The legal institution of human chattel slavery, comprising the enslavement primarily of Africans and African Americans, was prevalent in the United States of America from its founding in 1776 until 1865, predominantly in the South. Slavery was established throughout European colonization in the Americas. From 1526, during the early colonial period, it was practiced in what became Britain's colonies, including the Thirteen Colonies that formed the United States. Under the law, an enslaved person was treated as property that could be bought, sold, or given away. Slavery lasted in about half of U.S. states until abolition in 1865, and issues concerning slavery seeped into every aspect of national politics, economics, and social custom. In the decades after the end of Reconstruction in 1877, many of slavery's economic and social functions were continued through segregation, sharecropping, and convict leasing. Involuntary servitude as a punishment for crime is still legal in the United States.

By the time of the American Revolutionary War (1775–1783), the status of enslaved people had been institutionalized as a racial caste associated with African ancestry. During and immediately following the Revolution, abolitionist laws were passed in most Northern states and a movement developed to abolish slavery. The role of slavery under the United States Constitution (1789) was the most contentious issue during its drafting. The Three-Fifths Clause of the Constitution gave slave states disproportionate political power, while the Fugitive Slave Clause (Article IV, Section 2, Clause 3) provided that, if a slave escaped to another state, the other state could not prevent the return of the slave to the person claiming to be his or her owner. All Northern states had abolished slavery to some degree by 1805, sometimes with completion at a future date, sometimes with an intermediary status of unpaid indentured servant.

Abolition was in many cases a gradual process. Some slaveowners, primarily in the Upper South, freed their slaves, and charitable groups bought and freed others. The Atlantic slave trade began to be outlawed by individual states during the American Revolution. The import trade was banned by Congress in 1808, although smuggling was common thereafter, at which point the U.S. Revenue Cutter Service (Coast Guard) began enforcing the law on the high seas. It has been estimated that before 1820 a majority of serving congressmen owned slaves, and that about 30 percent of congressmen who were born before 1840 (some of whom served into the 20th century) at some time in their lives, were owners of slaves.

The rapid expansion of the cotton industry in the Deep South after the invention of the cotton gin greatly increased demand for slave labor, and the Southern states continued as slave societies. The U.S., divided into slave and free states, became ever more polarized over the issue of slavery. Driven by labor demands from new cotton plantations in the Deep South, the Upper South sold more than a million slaves who were taken to the Deep South. The total slave population in the South eventually reached four million. As the U.S. expanded, the Southern states attempted to extend slavery into the new Western territories to allow proslavery forces to maintain power in Congress. The new territories acquired by the Louisiana Purchase and the Mexican Cession were the subject of major political crises and compromises. Slavery was defended in the South as a "positive good", and the largest religious denominations split over the slavery issue into regional organizations of the North and South.

By 1850, the newly rich, cotton-growing South threatened to secede from the Union. Bloody fighting broke out over slavery in the Kansas Territory. When Abraham Lincoln won the 1860 election on a platform of halting the expansion of slavery, slave states seceded to form the Confederacy. Shortly afterward, the Civil War began when Confederate forces attacked the U.S. Army's Fort Sumter in Charleston, South Carolina. During the war some jurisdictions abolished slavery and, due to Union measures such as the Confiscation Acts and the Emancipation Proclamation, the war effectively ended slavery in most places. After the Union victory, the Thirteenth Amendment to the United States Constitution was ratified on December 6, 1865, prohibiting "slavery [and] involuntary servitude, except as a punishment for crime."

During most of the British colonial period, slavery existed in all the colonies. People enslaved in the North typically worked as house servants, artisans, laborers and craftsmen, with the greater number in cities. Many men worked on the docks and in shipping. In 1703, more than 42 percent of New York City households held enslaved people in bondage, the second-highest proportion of any city in the colonies, behind only Charleston, South Carolina. Enslaved people were also used as agricultural workers in farm communities, especially in the South, but also in upstate New York and Long Island, Connecticut, and New Jersey. By 1770, there were 397,924 blacks out of a population of 2.17 million in what would soon become the United States. The slaves of the colonial era were unevenly distributed: 14,867 lived in New England, where they were three percent of the population; 34,679 lived in the mid-Atlantic colonies, where they were six percent of the population; and 347,378 in the five Southern Colonies, where they were 31 percent of the population.

The South developed an agricultural economy dependent on commodity crops. Its planters rapidly acquired a significantly higher number and proportion of enslaved people in the population overall, as its commodity crops were labor-intensive. Early on, enslaved people in the South worked primarily on farms and plantations growing indigo, rice and tobacco (cotton did not become a major crop until after the 1790s). In 1720, about 65 percent of South Carolina's population was enslaved. Planters (defined by historians in the Upper South as those who held 20 or more slaves) used enslaved workers to cultivate commodity crops. They also worked in the artisanal trades on large plantations and in many Southern port cities. The later wave of settlers in the 18th century who settled along the Appalachian Mountains and backcountry were backwoods subsistence farmers, and they seldom held enslaved people.

Beginning in the second half of the 18th century, a debate emerged over the continued importation of African slaves to the American colonies. Many in the colonies, including the Southern slavocracy, opposed further importation of slaves due to fears that it would destabilize colonies and lead to further slave rebellions. In 1772, prominent Virginians submitted a petition to the Crown, requesting that the slave trade to Virginia be abolished; it was rejected. Rhode Island forbade the importation of slaves in 1774. The influential revolutionary Fairfax Resolves called for an end to the "wicked, cruel and unnatural" Atlantic slave trade. All of the colonies banned slave importations during the Revolutionary War.

Slavery had existed for thousands of years, all around the world. In the United States and many parts of the world it was a legal practise and had become entrenched socially and economically in many societies. The ideals and principles promoted in the Enlightenment and the American Revolution helped to put slavery and the desire for its abolition on the political agenda. As historian Christopher L. Brown put it, slavery "had never been on the agenda in a serious way before", but the American Revolution "forced it to be a public question from there forward".

After the new country's independence was secure, slavery was a topic of contention at the 1787 Constitutional Convention. Many of Founding Fathers of the United States were plantation owners who owned large numbers of enslaved laborers; the original Constitution preserved their right to own slaves, and they further gained a political advantage in owning slaves. Although the enslaved of the early Republic were considered sentient property, were not permitted to vote, and had no rights to speak of, they were to be enumerated in population censuses and counted as three-fifths of a person for the purposes of representation in the national legislature, the U.S. Congress.

The rebels began to offer freedom as an incentive to motivate slaves to fight on their side. Washington authorized slaves to be freed who fought with the American Continental Army. Rhode Island started enlisting slaves in 1778, and promised compensation to owners whose slaves enlisted and survived to gain freedom. During the course of the war, about one-fifth of the Northern army was black. In 1781, Baron Closen, a German officer in the French Royal Deux-Ponts Regiment at the Battle of Yorktown, estimated the American army to be about one-quarter black. These men included both former slaves and free-born blacks. Thousands of free blacks in the Northern states fought in the state militias and Continental Army. In the South, both sides offered freedom to slaves who would perform military service. Roughly 20,000 slaves fought in the American Revolution.

After the Revolutionary War broke out, the British realized they lacked the manpower necessary to prosecute the war. In response, British commanders began issuing proclamations to Patriot-owned slaves, offering freedom if they fled to British lines and assisted the British war effort. Such proclamations were repeatedly issued over the course of the conflict, which resulted in up to 100,000 American slaves fleeing to British lines. Self-emancipated slaves who reached British lines were organized into a variety of military units, which served in all theaters of the war. Formerly enslaved women and children, in lieu of military service, worked instead as laborers and domestic servants. At the end of the war, freed slaves in British lines either evacuated to other British colonies or to Britain itself, were re-enslaved by the victorious Americans, or fled into the countryside.

In early 1775, the royal governor of Virginia, Lord Dunmore, wrote to the Earl of Dartmouth of his intention to free slaves owned by American Patriots in case they staged a rebellion. On November 7, 1775, Dunmore issued Dunmore's Proclamation, which promised freedom to any slaves of American patriots who would leave their masters and join the British forces. Historians agree that the proclamation was chiefly designed for practical rather than moral reasons, and slaves owned by American Loyalists were unaffected by the proclamation. About 1,500 slaves owned by patriots escaped and joined Dunmore's forces. A total of 18 slaves fled George Washington's plantation, one of whom, Harry, served in Dunmore's all-black loyalist regiment called "the Black Pioneers". Escapees who joined Dunmore had "Liberty to Slaves" stitched on to their jackets. Most died of disease before they could do any fighting, but three hundred of these freed slaves made it to freedom in Britain. Historian Jill Lepore writes that "between eighty and a hundred thousand (nearly one in five black slaves) left their homes ... betting on British victory", but Cassandra Pybus states that between 20,000 and 30,000 is a more realistic number of slaves who defected to the British side during the war.

Many slaves took advantage of the disruption of war to escape from their plantations to British lines or to fade into the general population. Upon their first sight of British vessels, thousands of slaves in Maryland and Virginia fled from their owners. Throughout the South, losses of slaves were high, with many due to escapes. Slaves also escaped throughout New England and the mid-Atlantic, with many joining the British who had occupied New York. In the closing months of the war, the British evacuated freedmen and also removed slaves owned by loyalists. Around 15,000 black loyalists left with the British, most of them ending up as free people in England or its colonies. Washington hired a slave catcher during the war, and at its end he pressed the British to return the slaves to their masters. With the British certificates of freedom in their belongings, the black loyalists, including Washington's slave Harry, sailed with their white counterparts out of New York harbor to Nova Scotia. More than 3,000 were resettled in Nova Scotia, where they were eventually granted land and formed the community of the black Nova Scotians.

In the first two decades after the American Revolution, state legislatures and individuals took actions to free slaves. Northern states passed new constitutions that contained language about equal rights or specifically abolished slavery; some states, such as New York and New Jersey, where slavery was more widespread, passed laws by the end of the 18th century to abolish slavery incrementally. By 1804, all the Northern states had passed laws outlawing slavery, either immediately or over time. In New York, the last slaves were freed in 1827 (celebrated with a big July 5 parade). Indentured servitude, which had been widespread in the colonies (half the population of Philadelphia had once been indentured servants), dropped dramatically, and disappeared by 1800. However, there were still forcibly indentured servants in New Jersey in 1860. No Southern state abolished slavery, but some individual owners, more than a handful, freed their slaves by personal decision, often providing for manumission in wills but sometimes filing deeds or court papers to free individuals. Numerous slaveholders who freed their slaves cited revolutionary ideals in their documents; others freed slaves as a promised reward for service. From 1790 to 1810, the proportion of blacks free in the United States increased from 8 to 13.5 percent, and in the Upper South from less than one to nearly ten percent as a result of these actions.

Starting in 1777, the rebels outlawed the importation of slaves state by state. They all acted to end the international trade, but, after the war, it was reopened in North Carolina (opened until 1794) and Georgia (opened until 1798) and South Carolina (opened until 1787, and then reopened again in 1803.) In 1807, the United States Congress acted on President Thomas Jefferson's advice and, without controversy, made importing slaves from abroad a federal crime, effective the first day that the United States Constitution permitted this prohibition: January 1, 1808.

During the Revolution and in the following years, all states north of Maryland ( the Mason–Dixon line) took steps towards abolishing slavery. In 1777, the Vermont Republic, which was still unrecognized by the United States, passed a state constitution prohibiting slavery. The Pennsylvania Abolition Society, led in part by Benjamin Franklin, was founded in 1775, and Pennsylvania began gradual abolition in 1780. In 1783, the Supreme Judicial Court of Massachusetts ruled in Commonwealth v. Jennison that slavery was unconstitutional under the state's new 1780 constitution. New Hampshire began gradual emancipation in 1783, while Connecticut and Rhode Island followed suit in 1784. The New York Manumission Society, which was led by John Jay, Alexander Hamilton, and Aaron Burr, was founded in 1785. New York state began gradual emancipation in 1799, and New Jersey did the same in 1804.

Shortly after the Revolution, the Northwest Territory was established, by Manasseh Cutler and Rufus Putnam (who had been George Washington's chief engineer). Both Cutler and Putnam came from Puritan New England. The Puritans strongly believed that slavery was morally wrong. Their influence on the issue of slavery was long-lasting, and this was provided significantly greater impetus by the Revolution. The Northwest Territory (which became Ohio, Michigan, Indiana, Illinois, Wisconsin, and part of Minnesota) doubled the size of the United States, and it was established at the insistence of Cutler and Putnam as "free soil" – no slavery. This was to prove crucial a few decades later. Had those states been slave states, and their electoral votes gone to Abraham Lincoln's main opponent, Lincoln would not have become president. The Civil War would not have been fought. Even if it eventually had been, the North might well have lost.

Slavery was a contentious issue in the writing and approval of the Constitution of the United States. The words "slave" and "slavery" did not appear in the Constitution as originally adopted, although several provisions clearly referred to slaves and slavery. Until the adoption of the 13th Amendment in 1865, the Constitution did not prohibit slavery.

Section 9 of Article I forbade the federal government from prohibiting the importation of slaves, described as "such Persons as any of the States now existing shall think proper to admit", for twenty years after the Constitution's ratification (until January 1, 1808). The Act Prohibiting Importation of Slaves of 1807, passed by Congress and signed into law by President Thomas Jefferson (who had called for its enactment in his 1806 State of the Union address), went into effect on January 1, 1808, the earliest date on which the importation of slaves could be prohibited under the Constitution.

The delegates approved the Fugitive Slave Clause of the Constitution (Article IV, section 2, clause 3), which prohibited states from freeing those "held to Service or Labour" (meaning slaves, indentures, and apprentices) who fled to them from another state and required that they be returned to their owners. The Fugitive Slave Act of 1793 and the Fugitive Slave Act of 1850 gave effect to the Fugitive Slave Clause. Salmon P. Chase considered the Fugitive Slave Acts unconstitutional because "The Fugitive Slave Clause was a compact among the states, not a grant of power to the federal government".

In a section negotiated by James Madison of Virginia, Section 2 of Article I designated "other persons" (slaves) to be added to the total of the state's free population, at the rate of three-fifths of their total number, to establish the state's official population for the purposes of apportionment of congressional representation and federal taxation. The "Three-Fifths Compromise" was reached after a debate in which delegates from Southern (slaveholding) states argued that slaves should be counted in the census just as all other persons were while delegates from Northern (free) states countered that slaves should not be counted at all. The compromise strengthened the political power of Southern states, as three-fifths of the (non-voting) slave population was counted for congressional apportionment and in the Electoral College, although it did not strengthen Southern states as much as it would have had the Constitution provided for counting all persons, whether slave or free, equally.

In addition, many parts of the country were tied to the Southern economy. As the historian James Oliver Horton noted, prominent slaveholder politicians and the commodity crops of the South had a strong influence on United States politics and economy. Horton said,

in the 72 years between the election of George Washington and the election of Abraham Lincoln, 50 of those years [had] a slaveholder as president of the United States, and, for that whole period of time, there was never a person elected to a second term who was not a slaveholder.

The power of Southern states in Congress lasted until the Civil War, affecting national policies, legislation, and appointments. One result was that most of the justices appointed to the Supreme Court were slave owners. The planter elite dominated the Southern congressional delegations and the United States presidency for nearly fifty years.

Slavery in the United States was a variable thing, in "constant flux, driven by the violent pursuit of ever-larger profits." According to demographic calculations by J. David Hacker of the University of Minnesota, approximately four out of five of all of the slaves who ever lived in the United States or the territory that became the United States (beginning in 1619 and including all colonies that were eventually acquired or conquered by the United States) were born in or imported to the United States in the 19th century. Slaves were the labor force of the South, but slave ownership was also the foundation upon which American white supremacy was constructed. Historian Walter Johnson argues that "one of the many miraculous things a slave could do was make a household white...", meaning that the value of whiteness in America was in some ways measured by the ability to purchase and maintain black slaves.

Harriet Beecher Stowe described slavery in the United States in 1853:

What, then, is American slavery, as we have seen it exhibited by law, and by the decision of Courts? Let us begin by stating what it is not:

1. It is not apprenticeship.

2. It is not guardianship.

3. It is in no sense a system for the education of a weaker race by a stronger.

4. The happiness of the governed is in no sense its object.

5. The temporal improvement or the eternal well-being of the governed is in no sense its object.

The object of it has been distinctly stated in one sentence by Judge Ruffin,— "The end is the profit of the master, his security, and the public safety."

Slavery, then, is absolute despotism, of the most unmitigated form.

In the 19th century, proponents of slavery often defended the institution as a "necessary evil". At that time, it was feared that emancipation of black slaves would have more harmful social and economic consequences than the continuation of slavery. On April 22, 1820, Thomas Jefferson, one of the Founding Fathers of the United States, wrote in a letter to John Holmes, that with slavery,

We have the wolf by the ear, and we can neither hold him, nor safely let him go. Justice is in one scale, and self-preservation in the other.

The French writer and traveler Alexis de Tocqueville, in his influential Democracy in America (1835), expressed opposition to slavery while observing its effects on American society. He felt that a multiracial society without slavery was untenable, as he believed that prejudice against blacks increased as they were granted more rights (for example, in Northern states). He believed that the attitudes of white Southerners, and the concentration of the black population in the South, were bringing the white and black populations to a state of equilibrium, and were a danger to both races. Because of the racial differences between master and slave, he believed that the latter could not be emancipated.

In a letter to his wife dated December 27, 1856, in reaction to a message from President Franklin Pierce, Robert E. Lee wrote,

There are few, I believe, in this enlightened age, who will not acknowledge that slavery as an institution is a moral and political evil. It is idle to expatiate on its disadvantages. I think it is a greater evil to the white than to the colored race. While my feelings are strongly enlisted in behalf of the latter, my sympathies are more deeply engaged for the former. The blacks are immeasurably better off here than in Africa, morally, physically, and socially. The painful discipline they are undergoing is necessary for their further instruction as a race, and will prepare them, I hope, for better things. How long their servitude may be necessary is known and ordered by a merciful Providence.

However, as the abolitionist movement's agitation increased and the area developed for plantations expanded, apologies for slavery became more faint in the South. Leaders then described slavery as a beneficial scheme of labor management. John C. Calhoun, in a famous speech in the Senate in 1837, declared that slavery was "instead of an evil, a good – a positive good". Calhoun supported his view with the following reasoning: in every civilized society one portion of the community must live on the labor of another; learning, science, and the arts are built upon leisure; the African slave, kindly treated by his master and mistress and looked after in his old age, is better off than the free laborers of Europe; and under the slave system conflicts between capital and labor are avoided. The advantages of slavery in this respect, he concluded, "will become more and more manifest, if left undisturbed by interference from without, as the country advances in wealth and numbers".

South Carolina army officer, planter, and railroad executive James Gadsden called slavery "a social blessing" and abolitionists "the greatest curse of the nation". Gadsden was in favor of South Carolina's secession in 1850, and was a leader in efforts to split California into two states, one slave and one free.

Other Southern writers who also began to portray slavery as a positive good were James Henry Hammond and George Fitzhugh. They presented several arguments to defend the practice of slavery in the South. Hammond, like Calhoun, believed that slavery was needed to build the rest of society. In a speech to the Senate on March 4, 1858, Hammond developed his "Mudsill Theory", defending his view on slavery by stating: "Such a class you must have, or you would not have that other class which leads progress, civilization, and refinement. It constitutes the very mud-sill of society and of political government; and you might as well attempt to build a house in the air, as to build either the one or the other, except on this mud-sill." Hammond believed that in every class one group must accomplish all the menial duties, because without them the leaders in society could not progress. He argued that the hired laborers of the North were slaves too: "The difference ... is, that our slaves are hired for life and well compensated; there is no starvation, no begging, no want of employment," while those in the North had to search for employment.

George Fitzhugh used assumptions about white superiority to justify slavery, writing that, "the Negro is but a grown up child, and must be governed as a child." In The Universal Law of Slavery, Fitzhugh argues that slavery provides everything necessary for life and that the slave is unable to survive in a free world because he is lazy, and cannot compete with the intelligent European white race. He states that "The negro slaves of the South are the happiest, and in some sense, the freest people in the world." Without the South, "He (slave) would become an insufferable burden to society" and "Society has the right to prevent this, and can only do so by subjecting him to domestic slavery."

On March 21, 1861, Alexander Stephens, Vice President of the Confederacy, delivered his Cornerstone Speech. He explained the differences between the Constitution of the Confederate States and the United States Constitution, laid out the cause for the American Civil War, as he saw it, and defended slavery:

The new [Confederate] Constitution has put at rest forever all the agitating questions relating to our peculiar institutions – African slavery as it exists among us – the proper status of the negro in our form of civilization. This was the immediate cause of the late rupture and present revolution. Jefferson, in his forecast, had anticipated this, as the "rock upon which the old Union would split." He was right. What was conjecture with him, is now a realized fact. But whether he fully comprehended the great truth upon which that rock stood and stands, may be doubted. The prevailing ideas entertained by him and most of the leading statesmen at the time of the formation of the old Constitution were, that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally and politically. It was an evil they knew not well how to deal with; but the general opinion of the men of that day was, that, somehow or other, in the order of Providence, the institution would be evanescent and pass away ... Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the idea of a Government built upon it – when the "storm came and the wind blew, it fell".

Our new [Confederate] Government is founded upon exactly the opposite ideas; its foundations are laid, its cornerstone rests, upon the great truth that the negro is not equal to the white man; that slavery, subordination to the superior race, is his natural and moral condition.

This view of the "Negro race" was backed by pseudoscience. The leading researcher was Dr. Samuel A. Cartwright, a Southerner and the inventor of the mental illnesses of drapetomania (the desire of a slave to run away) and dysaesthesia aethiopica ("rascality"), both cured, according to him, by whipping. The Medical Association of Louisiana set up a committee, of which he was chair, to investigate "the Diseases and Physical Peculiarities of the Negro Race". Their report, first delivered to the Medical Association in an address, was published in their journal in 1851, and then reprinted in part in the widely circulated DeBow's Review.

Whether slavery was to be limited to the Southern states that already had it, or whether it was to be permitted in new states made from the lands of the Louisiana Purchase and Mexican Cession, was a major issue in the 1840s and 1850s. It was addressed by the Compromise of 1850 and during the Bleeding Kansas period.

Also relatively well-known are the proposals, including the Ostend Manifesto, to annex Cuba as a slave state, as well as the privately funded invasion of Cuba by Narciso López. There was also talk of making slave states of Mexico, Nicaragua (see Walker affair and Filibuster War) and other lands around the so-called Golden Circle. Less well known today, though well known at the time, is that pro-slavery Southerners:

• Actively sought to reopen the transatlantic slave trade
• Funded illegal slave shipments from the Caribbean and Africa, such as the Wanderer slave shipment to Georgia in 1858
• Wanted to reintroduce slavery in the Northern states, through federal action or Constitutional amendment making slavery legal nationwide, thus overriding state anti-slavery laws. (See Crittenden Compromise.) This was described as "well underway" by 1858.
• Said openly that slavery should by no means be limited to black people, since in their view it was beneficial. Northern white workers, who were allegedly "wage slaves" already, would allegedly have better lives if they were enslaved.

None of these ideas got very far, but they alarmed Northerners and contributed to the growing polarization of the country.

Slavery is a volcano, the fires of which cannot be quenched, nor its ravishes controlled. We already feel its convulsions, and if we sit idly gazing upon its flames, as they rise higher and higher, our happy republic will be buried in ruin, beneath its overwhelming energies.

— William Ellsworth, attorney for Prudence Crandall, 1834

Beginning during the Revolution and in the first two decades of the postwar era, every state in the North abolished slavery. These were the first abolitionist laws in the Atlantic World. However, the abolition of slavery did not necessarily mean that existing slaves became free. In some states they were forced to remain with their former owners as indentured servants: free in name only, although they could not be sold and thus families could not be split, and their children were born free. The end of slavery did not come in New York until July 4, 1827, when it was celebrated (on July 5) with a big parade. However, in the 1830 census, the only state with no slaves was Vermont. In the 1840 census, there were still slaves in New Hampshire (1), Rhode Island (5), Connecticut (17), New York (4), Pennsylvania (64), Ohio (3), Indiana (3), Illinois (331), Iowa (16), and Wisconsin (11). There were none in these states in the 1850 census.

Most Northern states passed legislation for gradual abolition, first freeing children born to slave mothers (and requiring them to serve lengthy indentures to their mother's owners, often into their 20s as young adults). In 1845, the Supreme Court of New Jersey received lengthy arguments towards "the deliverance of four thousand persons from bondage". Pennsylvania's last slaves were freed in 1847, Connecticut's in 1848, and while neither New Hampshire nor New Jersey had any slaves in the 1850 Census, and New Jersey only one and New Hampshire none in the 1860 Census, slavery was never prohibited in either state until ratification of the 13th Amendment in 1865 (and New Jersey was one of the last states to ratify it).

None of the Southern states abolished slavery before 1865, but it was not unusual for individual slaveholders in the South to free numerous slaves, often citing revolutionary ideals, in their wills. Methodist, Quaker, and Baptist preachers traveled in the South, appealing to slaveholders to manumit their slaves, and there were "manumission societies" in some Southern states. By 1810, the number and proportion of free blacks in the population of the United States had risen dramatically. Most free blacks lived in the North, but even in the Upper South, the proportion of free blacks went from less than one percent of all blacks to more than ten percent, even as the total number of slaves was increasing through imports.

African slaves arrived in the Massachusetts Bay Colony in the 1630s, and slavery was legally sanctioned by the Puritans in 1641. Massachusetts residents participated in the slave trade, and laws were passed regulating the movement and marriage among slaves. In 1700, Samuel Sewall, Puritan abolitionist and associate justice of the Massachusetts Superior Court of Judicature, wrote The Selling of Joseph, within which he condemned slavery and the slave trade and refuted many of the era's typical justifications for slavery. The Puritan influence on slavery was still strong at the time of the American Revolution and up until the Civil War. Of America's first seven presidents, the two who did not own slaves, John Adams and his son John Quincy Adams, came from Puritan New England. They were wealthy enough to own slaves, but they chose not to because they believed that it was morally wrong to do so. In 1765, colonial leader Samuel Adams and his wife were given a slave girl as a gift. They immediately freed her. Just after the Revolution, in 1787, the Northwest Territory (which became the states of Ohio, Michigan, Indiana, Illinois, Wisconsin and part of Minnesota) was opened up for settlement. The two men responsible for establishing this territory were Manasseh Cutler and Rufus Putnam. They came from Puritan New England, and they insisted that this new territory, which doubled the size of the United States, was going to be "free soil" – no slavery. This was to prove crucial in the coming decades. If those states had become slave states, and their electoral votes had gone to Abraham Lincoln's main opponent, Lincoln would not have been elected president.

In the decades leading up to the Civil War, the abolitionists, such as Theodore Parker, Ralph Waldo Emerson, Henry David Thoreau and Frederick Douglass, repeatedly used the Puritan heritage of the country to bolster their cause. The most radical anti-slavery newspaper, The Liberator, invoked the Puritans and Puritan values over a thousand times. Parker, in urging New England Congressmen to support the abolition of slavery, wrote that "The son of the Puritan ... is sent to Congress to stand up for Truth and Right ..."

Northerners predominated in the westward movement into the Midwestern territory after the American Revolution; as the states were organized, they voted to prohibit slavery in their constitutions when they achieved statehood: Ohio in 1803, Indiana in 1816, and Illinois in 1818. What developed was a Northern block of free states united into one contiguous geographic area that generally shared an anti-slavery culture. The exceptions were the areas along the Ohio River settled by Southerners: the southern portions of Indiana, Ohio, and Illinois. Residents of those areas generally shared in Southern culture and attitudes. In addition, these areas were devoted to agriculture longer than the industrializing northern parts of these states, and some farmers used slave labor. In Illinois, for example, while the trade in slaves was prohibited, it was legal to bring slaves from Kentucky into Illinois and use them there, as long as the slaves left Illinois one day per year (they were "visiting"). The emancipation of slaves in the North led to the growth in the population of Northern free blacks, from several hundred in the 1770s to nearly 50,000 by 1810.

Throughout the first half of the 19th century, abolitionism, a movement to end slavery, grew in strength; most abolitionist societies and supporters were in the North. They worked to raise awareness about the evils of slavery, and to build support for abolition. After 1830, abolitionist and newspaper publisher William Lloyd Garrison promoted emancipation, characterizing slaveholding as a personal sin. He demanded that slaveowners repent and start the process of emancipation. His position increased defensiveness on the part of some Southerners, who noted the long history of slavery among many cultures. A few abolitionists, such as John Brown, favored the use of armed force to foment uprisings among the slaves, as he attempted to do at Harper's Ferry. Most abolitionists tried to raise public support to change laws and to challenge slave laws. Abolitionists were active on the lecture circuit in the North, and often featured escaped slaves in their presentations. Writer and orator Frederick Douglass became an important abolitionist leader after escaping from slavery. Harriet Beecher Stowe's novel Uncle Tom's Cabin (1852) was an international bestseller, and along with the non-fiction companion A Key to Uncle Tom's Cabin, aroused popular sentiment against slavery. It also provoked the publication of numerous anti-Tom novels by Southerners in the years before the American Civil War.

This struggle took place amid strong support for slavery among white Southerners, who profited greatly from the system of enslaved labor. But slavery was entwined with the national economy; for instance, the banking, shipping, insurance, and manufacturing industries of New York City all had strong economic interests in slavery, as did similar industries in other major port cities in the North. The Northern textile mills in New York and New England processed Southern cotton and manufactured clothes to outfit slaves. By 1822, half of New York City's exports were related to cotton.

Slaveholders began to refer to slavery as the "peculiar institution" to differentiate it from other examples of forced labor. They justified it as less cruel than the free labor of the North.

The principal organized bodies to advocate abolition and anti-slavery reforms in the north were the Pennsylvania Abolition Society and the New York Manumission Society. Before the 1830s the antislavery groups called for gradual emancipation. By the late 1820s, under the impulse of religious evangelicals such as Beriah Green, the sense emerged that owning slaves was a sin and the owner had to immediately free himself from this grave sin by immediate emancipation.

Under the Constitution, Congress could not prohibit the import slave trade that was allowed in South Carolina until 1808. However, the third Congress regulated against it in the Slave Trade Act of 1794, which prohibited American shipbuilding and outfitting for the trade. Subsequent acts in 1800 and 1803 sought to discourage the trade by banning American investment in the trade, and American employment on ships in the trade, as well as prohibiting importation into states that had abolished slavery, which all states except South Carolina had by 1807. The final Act Prohibiting Importation of Slaves was adopted in 1807 and went into effect in 1808. However, illegal importation of African slaves (smuggling) was common. The Cuban slave trade between 1796 and 1807 was dominated by American slave ships. Despite the 1794 Act, Rhode Island slave ship owners found ways to continue supplying the slave-owning states. The overall U.S. slave-ship fleet in 1806 was estimated to be almost 75% the size of that of the British.

After Great Britain and the United States outlawed the international slave trade in 1807, British slave trade suppression activities began in 1808 through diplomatic efforts and the formation of the Royal Navy's West Africa Squadron in 1809. The United States denied the Royal Navy the right to stop and search U.S. ships suspected as slave ships, so not only were American ships unhindered by British patrols, but slavers from other countries would fly the American flag to try to avoid being stopped. Co-operation between the United States and Britain was not possible during the War of 1812 or the period of poor relations in the following years. In 1820, the United States Navy sent USS Cyane under the command of Captain Edward Trenchard to patrol the slave coasts of West Africa. Cyane seized four American slave ships in her first year on station. Trenchard developed a good level of co-operation with the Royal Navy. Four additional U.S. warships were sent to the African coast in 1820 and 1821. A total of 11 American slave ships were taken by the U.S. Navy over this period. Then American enforcement activity reduced. There was still no agreement between the United States and Britain on a mutual right to board suspected slave traders sailing under each other's flag. Attempts to reach such an agreement stalled in 1821 and 1824 in the United States Senate. A U.S. Navy presence, however sporadic, did result in American slavers sailing under the Spanish flag, but still as an extensive trade. The Webster-Ashburton Treaty of 1842 set a guaranteed minimum level of patrol activity by the U.S. Navy and the Royal Navy, and formalized the level of co-operation that had existed in 1820. Its effects, however, were minimal while opportunities for greater co-operation were not taken. The U.S. transatlantic slave trade was not effectively suppressed until 1861, during Lincoln's presidency, when a treaty with Britain was signed whose provisions included allowing the Royal Navy to board, search and arrest slavers operating under the American flag.

During the War of 1812, British Royal Navy commanders of the blockading fleet were instructed to offer freedom to defecting American slaves, as the Crown had during the Revolutionary War. Thousands of escaped slaves went over to the Crown with their families. Men were recruited into the Corps of Colonial Marines on occupied Tangier Island, in the Chesapeake Bay. Many freed American slaves were recruited directly into existing West Indian regiments, or newly created British Army units. The British later resettled a few thousand freed slaves to Nova Scotia. Their descendants, together with descendants of the black people resettled there after the Revolution, have established the Black Loyalist Heritage Museum.

Slaveholders, primarily in the South, had considerable "loss of property" as thousands of slaves escaped to the British lines or ships for freedom, despite the difficulties. The planters' complacency about slave "contentment" was shocked by seeing that slaves would risk so much to be free. Afterward, when some freed slaves had been settled at Bermuda, slaveholders such as Major Pierce Butler of South Carolina tried to persuade them to return to the United States, to no avail.

The Americans protested that Britain's failure to return all slaves violated the Treaty of Ghent. After arbitration by the Tsar of Russia, the British paid $1,204,960 in damages (about $32.4 million in today's money) to Washington, which reimbursed the slaveowners.

According to Herbert Aptheker, "there were few phases of ante-bellum Southern life and history that were not in some way influenced by the fear of, or the actual outbreak of, militant concerted slave action."

Historians in the 20th century identified 250 to 311 slave uprisings in U.S. and colonial history. Those after 1776 include:

• Gabriel's conspiracy (1800)
• Igbo Landing slave escape and mass suicide (1803)
• Chatham Manor Rebellion (1805)
• 1811 German Coast uprising, (1811)
• George Boxley Rebellion (1815)
• Denmark Vesey's conspiracy (1822)
• Nat Turner's Rebellion (1831)
• Black Seminole Slave Rebellion (1835–1838)
• Amistad seizure (1839)
• 1842 Slave Revolt in the Cherokee Nation
• Charleston Workhouse Slave Rebellion (1849)

In 1831, Nat Turner, a literate slave who claimed to have spiritual visions, organized a slave rebellion in Southampton County, Virginia; it was sometimes called the Southampton Insurrection. Turner and his followers killed nearly sixty white inhabitants, mostly women and children. Many of the men in the area were attending a religious event in North Carolina. Eventually Turner was captured with 17 other rebels, who were subdued by the militia. Turner and his followers were hanged, and Turner's body was flayed. In a frenzy of fear and retaliation, the militia killed more than 100 slaves who had not been involved in the rebellion. Planters whipped hundreds of innocent slaves to ensure resistance was quelled.

This rebellion prompted Virginia and other slave states to pass more restrictions on slaves and free people of color, controlling their movement and requiring more white supervision of gatherings. In 1835, North Carolina withdrew the franchise for free people of color, and they lost their vote.

There are four known mutinies on vessels involved in the coastwise slave trade: Decatur (1826), Governor Strong (1826), Lafayette (1829), and the Creole (1841).

Although Virginia, Maryland and Delaware were slave states, the latter two already had a high proportion of free blacks by the outbreak of war. Following the Revolution, the three legislatures made manumission easier, allowing it by deed or will. Quaker and Methodist ministers in particular urged slaveholders to free their slaves. The number and proportion of freed slaves in these states rose dramatically until 1810. More than half of the number of free blacks in the United States were concentrated in the Upper South. The proportion of free blacks among the black population in the Upper South rose from less than 1 percent in 1792 to more than 10 percent by 1810. In Delaware, nearly 75 percent of black people were free by 1810.

In the United States as a whole, the number of free blacks reached 186,446, or 13.5 percent of all black people by 1810. After that period, few slaves were freed, as the development of cotton plantations featuring short-staple cotton in the Deep South drove up the internal demand for slaves in the domestic slave trade and high prices being paid for them.

South Carolina made manumission more difficult, requiring legislative approval of every manumission. Alabama banned free black people from the state beginning in 1834; free people of color who crossed the state line were subject to enslavement. Free black people in Arkansas after 1843 had to buy a $500 good-behavior bond, and no unenslaved black person was legally allowed to move into the state.

Women exercised their right to own and control human property without their husbands' interference or permission, and they were active participants in the slave trade. For example, in South Carolina 40% of bills of sale for slaves from the 1700s to the present included a female buyer or seller. Women also governed their slaves in a manner similar to men, engaging in the same levels of physical disciplining. Like men, they brought lawsuits against those who jeopardized their ownership to their slaves.

Despite the longstanding color line in the United States, some African Americans were slave owners themselves, some in cities and others as plantation owners in the country. Slave ownership signified both wealth and increased social status. Black slave owners were uncommon, however, as "of the two and a half million African Americans living in the United States in 1850, the vast majority [were] enslaved."

After 1800, some of the Cherokee and the other four civilized tribes of the Southeast started buying and using black slaves as labor. They continued this practice after removal to Indian Territory in the 1830s, when as many as 15,000 enslaved blacks were taken with them.

The nature of slavery in Cherokee society often mirrored that of white slave-owning society. The law barred intermarriage of Cherokees and enslaved African Americans, but Cherokee men had unions with enslaved women, resulting in mixed-race children. Cherokee who aided slaves were punished with one hundred lashes on the back. In Cherokee society, persons of African descent were barred from holding office even if they were also racially and culturally Cherokee. They were also barred from bearing arms and owning property. The Cherokee prohibited the teaching of African Americans to read and write.

By contrast, the Seminole welcomed into their nation African Americans who had escaped slavery (Black Seminoles). Historically, the Black Seminoles lived mostly in distinct bands near the Native American Seminole. Some were held as slaves of particular Seminole leaders. Seminole practice in Florida had acknowledged slavery, though not the chattel slavery model common elsewhere. It was, in fact, more like feudal dependency and taxation. The relationship between Seminole blacks and natives changed following their relocation in the 1830s to territory controlled by the Creek who had a system of chattel slavery. Pro slavery pressure from Creek and pro-Creek Seminole and slave raiding led to many Black Seminoles escaping to Mexico.

The United States Constitution, adopted in 1787, prevented Congress from completely banning the importation of slaves until 1808, although Congress regulated against the trade in the Slave Trade Act of 1794, and in subsequent Acts in 1800 and 1803. During and after the Revolution, the states individually passed laws against importing slaves. By contrast, the states of Georgia and South Carolina reopened their trade due to demand by their upland planters, who were developing new cotton plantations: Georgia from 1800 until December 31, 1807, and South Carolina from 1804. In that period, Charleston traders imported about 75,000 slaves, more than were brought to South Carolina in the 75 years before the Revolution. Approximately 30,000 were imported to Georgia.

By January 1, 1808, when Congress banned further imports, South Carolina was the only state that still allowed importation of enslaved people. The domestic trade became extremely profitable as demand rose with the expansion of cultivation in the Deep South for cotton and sugar cane crops. Slavery in the United States became, more or less, self-sustaining by natural increase among the current slaves and their descendants. Maryland and Virginia viewed themselves as slave producers, seeing "producing slaves" as resembling animal husbandry. Workers, including many children, were relocated by force from the upper to the lower South.

Despite the ban, slave imports continued through smugglers bringing in slaves past the U.S. Navy's African Slave Trade Patrol to South Carolina, and overland from Texas and Florida, both under Spanish control. Congress increased the punishment associated with importing slaves, classifying it in 1820 as an act of piracy, with smugglers subject to harsh penalties, including death if caught. After that, "it is unlikely that more than 10,000 [slaves] were successfully landed in the United States." But, some smuggling of slaves into the United States continued until just before the start of the Civil War.

In the early part of the 19th century, other organizations were founded to take action on the future of black Americans. Some advocated removing free black people from the United States to places where they would enjoy greater freedom; some endorsed colonization in Africa, while others advocated emigration, usually to Haiti. During the 1820s and 1830s, the American Colonization Society (ACS) was the primary organization to implement the "return" of black Americans to Africa. The ACS was made up mostly of Quakers and slaveholders, and they found uneasy common ground in support of what was incorrectly called "repatriation". By this time, however, most black Americans were native-born and did not want to emigrate, saying they were no more African than white Americans were British. Rather, they wanted full rights in the United States, where their families had lived and worked for generations.

In 1822, the ACS and affiliated state societies established what would become the colony of Liberia, in West Africa. The ACS assisted thousands of freedmen and free blacks (with legislated limits) to emigrate there from the United States. Many white people considered this preferable to emancipation in the United States. Henry Clay, one of the founders and a prominent slaveholder politician from Kentucky, said that blacks faced:

...unconquerable prejudice resulting from their color, they never could amalgamate with the free whites of this country. It was desirable, therefore, as it respected them, and the residue of the population of the country, to drain them off.

Deportation would also be a way to prevent reprisals against former slaveholders and white people in general, as had occurred in the 1804 Haiti massacre, which had contributed to a consuming fear amongst whites of retributive black violence, a phobia dubbed Haitianism.

The U.S. Constitution barred the federal government from prohibiting the importation of slaves for twenty years. Various states passed bans on the international slave trade during that period; by 1808, the only state still allowing the importation of African slaves was South Carolina. After 1808, legal importation of slaves ceased, although there was smuggling via Spanish Florida and the disputed Gulf Coast to the west. This route all but ended after Florida became a U.S. territory in 1821 (but see slave ships Wanderer and Clotilda).

The replacement for the importation of slaves from abroad was increased domestic production. Virginia and Maryland had little new agricultural development, and their need for slaves was mostly for replacements for decedents. Normal reproduction more than supplied these: Virginia and Maryland had surpluses of slaves. Their tobacco farms were "worn out" and the climate was not suitable for cotton or sugar cane. The surplus was even greater because slaves were encouraged to reproduce (though they could not marry). The pro-slavery Virginian Thomas Roderick Dew wrote in 1832 that Virginia was a "negro-raising state"; i.e. Virginia "produced" slaves. According to him, in 1832 Virginia exported "upwards of 6,000 slaves" per year, "a source of wealth to Virginia". A newspaper from 1836 gives the figure as 40,000, earning for Virginia an estimated $24,000,000 per year. Demand for slaves was the strongest in what was then the southwest of the country: Alabama, Mississippi, and Louisiana, and, later, Texas, Arkansas, and Missouri. Here there was abundant land suitable for plantation agriculture, which young men with some capital established. This was expansion of the white, monied population: younger men seeking their fortune.

The most valuable crop that could be grown on a plantation in that climate was cotton. That crop was labor-intensive, and the least-costly laborers were slaves. Demand for slaves exceeded the supply in the southwest; therefore slaves, never cheap if they were productive, went for a higher price. As portrayed in Uncle Tom's Cabin (the "original" cabin was in Maryland), "selling South" was greatly feared. A recently (2018) publicized example of the practice of "selling South" is the 1838 sale by Jesuits of 272 slaves from Maryland, to plantations in Louisiana, to benefit Georgetown University, which has been described as "ow[ing] its existence" to this transaction.

The growing international demand for cotton led many plantation owners further west in search of suitable land. In addition, the invention of the cotton gin in 1793 enabled profitable processing of short-staple cotton, which could readily be grown in the uplands. The invention revolutionized the cotton industry by increasing fifty-fold the quantity of cotton that could be processed in a day. At the end of the War of 1812, fewer than 300,000 bales of cotton were produced nationally. By 1820, the amount of cotton produced had increased to 600,000 bales, and by 1850 it had reached 4,000,000. There was an explosive growth of cotton cultivation throughout the Deep South and greatly increased demand for slave labor to support it. As a result, manumissions decreased dramatically in the South.

Most of the slaves sold from the Upper South were from Maryland, Virginia and the Carolinas, where changes in agriculture decreased the need for their labor and the demand for slaves. Before 1810, primary destinations for the slaves who were sold were Kentucky and Tennessee, but, after 1810, the Deep South states of Georgia, Alabama, Mississippi, Louisiana and Texas received the most slaves. This is where cotton became "king". Meanwhile, the Upper South states of Kentucky and Tennessee joined the slave-exporting states.

By 1815, the domestic slave trade had become a major economic activity in the United States; it lasted until the 1860s. Between 1830 and 1840, nearly 250,000 slaves were taken across state lines. In the 1850s, more than 193,000 enslaved persons were transported, and historians estimate nearly one million in total took part in the forced migration of this new "Middle Passage". By 1860, the slave population in the United States had reached four million. Of the 1,515,605 free families in the fifteen slave states in 1860, nearly 400,000 held slaves (roughly one in four, or 25%), amounting to 8% of all American families.

The historian Ira Berlin called this forced migration of slaves the "Second Middle Passage" because it reproduced many of the same horrors as the Middle Passage (the name given to the transportation of slaves from Africa to North America). These sales of slaves broke up many families and caused much hardship. Characterizing it as the "central event" in the life of a slave between the American Revolution and the Civil War, Berlin wrote that, whether slaves were directly uprooted or lived in fear that they or their families would be involuntarily moved, "the massive deportation traumatized black people, both slave and free". Individuals lost their connection to families and clans. Added to the earlier colonists combining slaves from different tribes, many ethnic Africans lost their knowledge of varying tribal origins in Africa. Most were descended from families that had been in the United States for many generations.

The firm of Franklin and Armfield was a leader in this trade. In the 1840s, almost 300,000 slaves were transported, with Alabama and Mississippi receiving 100,000 each. During each decade between 1810 and 1860, at least 100,000 slaves were moved from their state of origin. In the final decade before the Civil War, 250,000 were transported. Michael Tadman wrote in Speculators and Slaves: Masters, Traders, and Slaves in the Old South (1989) that 60–70% of inter-regional migrations were the result of the sale of slaves. In 1820, a slave child in the Upper South had a 30 percent chance of being sold South by 1860. The death rate for the slaves on their way to their new destination across the American South was less than that suffered by captives shipped across the Atlantic Ocean, but mortality nevertheless was higher than the normal death rate.

Slave traders transported two-thirds of the slaves who moved West. Only a minority moved with their families and existing master. Slave traders had little interest in purchasing or transporting intact slave families; in the early years, planters demanded only the young male slaves needed for heavy labor. Later, in the interest of creating a "self-reproducing labor force", planters purchased nearly equal numbers of men and women. Berlin wrote:

The internal slave trade became the largest enterprise in the South outside the plantation itself, and probably the most advanced in its employment of modern transportation, finance, and publicity. The slave trade industry developed its own unique language, with terms such as "prime hands, bucks, breeding wenches, and "fancy girls" coming into common use.

The expansion of the interstate slave trade contributed to the "economic revival of once depressed seaboard states" as demand accelerated the value of slaves who were subject to sale. Some traders moved their "chattels" by sea, with Norfolk to New Orleans being the most common route, but most slaves were forced to walk overland. Others were shipped downriver from such markets as Louisville on the Ohio River, and Natchez on the Mississippi. Traders created regular migration routes served by a network of slave pens, yards and warehouses needed as temporary housing for the slaves. In addition, other vendors provided clothes, food and supplies for slaves. As the trek advanced, some slaves were sold and new ones purchased. Berlin concluded, "In all, the slave trade, with its hubs and regional centers, its spurs and circuits, reached into every cranny of southern society. Few southerners, black or white, were untouched."

Once the trip ended, slaves faced a life on the frontier significantly different from most labor in the Upper South. Clearing trees and starting crops on virgin fields was harsh and backbreaking work. A combination of inadequate nutrition, bad water and exhaustion from both the journey and the work weakened the newly arrived slaves and produced casualties. New plantations were located at rivers' edges for ease of transportation and travel. Mosquitoes and other environmental challenges spread disease, which took the lives of many slaves. They had acquired only limited immunities to lowland diseases in their previous homes. The death rate was so high that, in the first few years of hewing a plantation out of the wilderness, some planters preferred whenever possible to use rented slaves rather than their own.

The harsh conditions on the frontier increased slave resistance and led owners and overseers to rely on violence for control. Many of the slaves were new to cotton fields and unaccustomed to the "sunrise-to-sunset gang labor" required by their new life. Slaves were driven much harder than when they had been in growing tobacco or wheat back East. Slaves had less time and opportunity to improve the quality of their lives by raising their own livestock or tending vegetable gardens, for either their own consumption or trade, as they could in the East.

In Louisiana, French colonists had established sugar cane plantations and exported sugar as the chief commodity crop. After the Louisiana Purchase in 1803, Americans entered the state and joined the sugar cultivation. Between 1810 and 1830, planters bought slaves from the North and the number of slaves increased from fewer than 10,000 to more than 42,000. Planters preferred young males, who represented two-thirds of the slave purchases. Dealing with sugar cane was even more physically demanding than growing cotton. The largely young, unmarried male slave force made the reliance on violence by the owners "especially savage".

New Orleans became nationally important as a slave market and port, as slaves were shipped from there upriver by steamboat to plantations on the Mississippi River; it also sold slaves who had been shipped downriver from markets such as Louisville. By 1840, the New Orleans slave market was the largest in North America. It became the wealthiest and the fourth-largest city in the nation, based chiefly on the slave trade and associated businesses. The trading season was from September to May, after the harvest.

The notion that slave traders were social outcasts of low reputation, even in the South, was initially promulgated by defensive southerners and later by figures like historian Ulrich B. Phillips. Historian Frederic Bancroft, author of Slave-Trading in the Old South (1931) found — to the contrary of Phillips's position — that many traders were esteemed members of their communities. Contemporary researcher Steven Deyle argues that the "trader's position in society was not unproblematic and owners who dealt with the trader felt the need to satisfy themselves that they acted honorably," while Michael Tadman contends that "'trader as outcast' operated at the level of propaganda" whereas white slave owners almost universally professed a belief that slaves were not human like them, and thus dismissed the consequences of slave trading as beneath consideration. Similarly, historian Charles Dew read hundreds of letters to slave traders and found virtually zero narrative evidence for guilt, shame, or contrition about the slave trade: "If you begin with the absolute belief in white supremacy—unquestioned white superiority/unquestioned black inferiority—everything falls neatly into place: the African is inferior racial 'stock,' living in sin and ignorance and barbarism and heathenism on the 'Dark Continent' until enslaved...Slavery thus miraculously becomes a form of 'uplift' for this supposedly benighted and brutish race of people. And once notions of white supremacy and black inferiority are in place in the American South, they are passed on from one generation to the next with all the certainty and inevitability of a genetic trait."

In the 1828 presidential election, candidate Andrew Jackson was strongly criticized by opponents as a slave trader who transacted in slaves in defiance of modern standards or morality.

The treatment of slaves in the United States varied widely depending on conditions, time, and place, but in general it was brutal, especially on plantations. Whippings and rape were routine. The power relationships of slavery corrupted many whites that had authority over slaves, with children showing their own cruelty. Masters and overseers resorted to physical punishments to impose their wills. Slaves were punished by whipping, shackling, hanging, beating, burning, mutilation, branding and imprisonment. Punishment was most often meted out in response to disobedience or perceived infractions, but sometimes abuse was carried out to re-assert the dominance of the master or overseer of the slave. Treatment was usually harsher on large plantations, which were often managed by overseers and owned by absentee slaveholders, conditions permitting abuses.

William Wells Brown, who escaped to freedom, reported that on one plantation, slave men were required to pick eighty pounds per day of cotton, while women were required to pick seventy pounds; if any slave failed in his or her quota, they were subject to whip lashes for each pound they were short. The whipping post stood next to the cotton scales. A New York man who attended a slave auction in the mid-19th century reported that at least three-quarters of the male slaves he saw at sale had scars on their backs from whipping. By contrast, small slave-owning families had closer relationships between the owners and slaves; this sometimes resulted in a more humane environment but was not a given.

Historian Lawrence M. Friedman wrote: "Ten Southern codes made it a crime to mistreat a slave. ... Under the Louisiana Civil Code of 1825 (art. 192), if a master was "convicted of cruel treatment", the judge could order the sale of the mistreated slave, presumably to a better master. Masters and overseers were seldom prosecuted under these laws. No slave could give testimony in the courts.

According to Adalberto Aguirre's research, 1,161 slaves were executed in the United States between the 1790s and 1850s. Quick executions of innocent slaves as well as suspects typically followed any attempted slave rebellions, as white militias overreacted with widespread killings that expressed their fears of rebellions, or suspected rebellions.

Although most slaves had lives that were very restricted in terms of their movements and agency, exceptions existed to virtually every generalization; for instance, there were also slaves who had considerable freedom in their daily lives: slaves allowed to rent out their labor and who might live independently of their master in cities, slaves who employed white workers, and slave doctors who treated upper-class white patients. After 1820, in response to the inability to import new slaves from Africa and in part to abolitionist criticism, some slaveholders improved the living conditions of their slaves, to encourage them to be productive and to try to prevent escapes. It was part of a paternalistic approach in the antebellum era that was encouraged by ministers trying to use Christianity to improve the treatment of slaves. Slaveholders published articles in Southern agricultural journals to share best practices in treatment and management of slaves; they intended to show that their system was better than the living conditions of northern industrial workers.

Medical care for slaves was limited in terms of the medical knowledge available to anyone. It was generally provided by other slaves or by slaveholders' family members, although sometimes "plantation physicians", like J. Marion Sims, were called by the owners to protect their investment by treating sick slaves. Many slaves possessed medical skills needed to tend to each other, and used folk remedies brought from Africa. They also developed new remedies based on American plants and herbs.

An estimated nine percent of slaves were disabled due to a physical, sensory, psychological, neurological, or developmental condition. However, slaves were often described as disabled if they were unable to work or bear a child, and were often subjected to harsh treatment as a result.

According to Andrew Fede, an owner could be held criminally liable for killing a slave only if the slave he killed was "completely submissive and under the master's absolute control". For example, in 1791 the North Carolina General Assembly defined the willful killing of a slave as criminal murder, unless done in resisting or under moderate correction (that is, corporal punishment).

While slaves' living conditions were poor by modern standards, Robert Fogel argued that all workers, free or slave, during the first half of the 19th century were subject to hardship. Unlike free individuals, however, enslaved people were far more likely to be underfed, physically punished, sexually abused, or killed, with no recourse, legal or otherwise, against those who perpetrated these crimes against them.

In a very grim fashion, the commodification of the human body was legal in the case of African slaves as they were not legally seen as fully human. The most popular means of commodifying slave tissues was through medical experimentation. Slaves were routinely used as medical specimens forced to take part in experimental surgeries, amputations, disease research, and developing medical techniques. Many slaves in these routine experiments were not given pain relief or analgesics, resulting in death by shock on the table. The bodies of such slaves were grouped with other medical cadavers, or sold with the bodies of other slaves sold, stolen, or grave robbed for medical experimentation. In many cases, slave cadavers were used in demonstrations and dissection tables, oftentimes resulting in their tissues being sold for profit.

For the reason of slave punishment, decoration, or self-expression, the skin of slaves was in many instances allowed to be made into leather for furniture, accessories, and clothing, a common instance of which being that of wealthy clientele sending cadaver skin to tanners and shoemakers under the guise of animal leather. Slave hair could be shaved and used for stuffing in pillows and furniture. In some instances, the inner body tissue of slaves (fat, bones, etc.) could be made into soap, medicinal grease, trophies, and other commodities.

Because of the power relationships at work, slave women in the United States were at high risk for rape and sexual abuse. Their children were repeatedly taken away from them and sold as chattel; usually they never saw each other again. Many slaves fought back against sexual attacks, and some died resisting. Others carried psychological and physical scars from the attacks. Sexual abuse of slaves was partially rooted in a patriarchal Southern culture that treated black women as property or chattel. Southern culture strongly policed against sexual relations between white women and black men on the purported grounds of racial purity but, by the late 18th century, the many mixed-race slaves and slave children showed that white men had often taken advantage of slave women. Wealthy planter widowers, notably such as John Wayles and his son-in-law Thomas Jefferson, took slave women as concubines; each had six children with his partner: Elizabeth Hemings and her daughter Sally Hemings (the half-sister of Jefferson's late wife), respectively. Both Mary Chesnut and Fanny Kemble, wives of planters, wrote about this issue in the antebellum South in the decades before the Civil War. Sometimes planters used mixed-race slaves as house servants or favored artisans because they were their children or other relatives. While publicly opposed to race mixing, in his Notes on the State of Virginia published in 1785, Jefferson wrote: "The improvement of the blacks in body and mind, in the first instance of their mixture with the whites, has been observed by every one, and proves that their inferiority is not the effect merely of their condition of life". Historians estimate that 58% of enslaved women in the U.S. aged 15–30 years were sexually assaulted by their slave owners and other white men. As a result of centuries of slavery and such relationships, DNA studies have shown that the vast majority of African Americans also have historic European ancestry, generally through paternal lines. The average Black American genome is roughly 20-25% European, and it is estimated that as much as one third of their Y chromosomes are of European origin.

Portrayals of black men as hypersexual and savage, along with ideals of protecting white women, were predominant during this time and masked the experiences of sexual violence faced by black male slaves, especially by white women. Subject not only to rape and sexual exploitation, slaves faced sexual violence in many forms. A black man could be forced by his slaveowner to rape another slave or even a free black woman. Forced pairings with other slaves, including forced breeding, which neither slave might desire, were common. Despite explicit bans on homosexuality and sodomy, it was not uncommon for male slaves and children to be sexually harassed and assaulted by their masters in secret. Through sexual and reproductive abuse slaveowners could further enforce their control over their slaves.

The prohibition on the importation of slaves into the United States after 1808 limited the supply of slaves in the United States. This came at a time when the invention of the cotton gin enabled the expansion of cultivation in the uplands of short-staple cotton, leading to clearing lands cultivating cotton through large areas of the Deep South, especially the Black Belt. The demand for labor in the area increased sharply and led to an expansion of the internal slave market. At the same time, the Upper South had an excess number of slaves because of a shift to mixed-crops agriculture, which was less labor-intensive than tobacco. To add to the supply of slaves, slaveholders looked at the fertility of slave women as part of their productivity, and intermittently forced the women to have large numbers of children. During this time period, the terms "breeders", "breeding slaves", "child bearing women", "breeding period", and "too old to breed" became familiar.

As it became popular on many plantations to breed slaves for strength, fertility, or extra labor, there grew many documented instances of "breeding farms" in the United States. Slaves were forced to conceive and birth as many new slaves as possible. The largest farms were located in Virginia and Maryland. Because the industry of slave breeding came from a desire for larger than natural population growth of slaves, slaveowners often turned towards systematic practices for creating more slaves. Female slaves "were subjected to repeated rape or forced sex and became pregnant again and again", even by incest. In horrific accounts of former slaves, some stated that hoods or bags were placed over their heads to prevent them from knowing who they were forced to have sex with. Journalist William Spivey wrote, "It could be someone they know, perhaps a niece, aunt, sister, or their own mother. The breeders only wanted a child that could be sold."

In the United States in the early 19th century, owners of female slaves could freely and legally use them as sexual objects. This follows free use of female slaves on slaving vessels by the crews.

The slaveholder has it in his power, to violate the chastity of his slaves. And not a few are beastly enough to exercise such power. Hence it happens that, in some families, it is difficult to distinguish the free children from the slaves. It is sometimes the case, that the largest part of the master's own children are born, not of his wife, but of the wives and daughters of his slaves, whom he has basely prostituted as well as enslaved.

"This vice, this bane of society, has already become so common, that it is scarcely esteemed a disgrace."

"Fancy" was a code word that indicated that the girl or young woman was suitable for or trained for sexual use. In some cases, children were also abused in this manner. The sale of a 13-year-old "nearly a fancy" is documented. Zephaniah Kingsley, Jr., bought his wife when she was 13.

Furthermore, enslaved women who were old enough to bear children were encouraged to procreate, which raised their value as slaves, since their children would eventually provide labor or be sold, enriching the owners. Enslaved women were sometimes medically treated to enable or encourage their fertility. The variations in skin color found in the United States make it obvious how often black women were impregnated by whites. For example, in the 1850 Census, 75.4% of "free negros" in Florida were described as mulattos, of mixed race. Nevertheless, it is only very recently, with DNA studies, that any sort of reliable number can be provided, and the research has only begun. Light-skinned girls, who contrasted with the darker field workers, were preferred.

As Caroline Randall Williams was quoted in The New York Times: "You Want a Confederate Monument? My Body Is a Confederate Monument." "I have rape-colored skin", she added.

The sexual use of black slaves by either slave owners or by those who could purchase the temporary services of a slave took various forms. A slaveowner, or his teenage son, could go to the slave quarters area of the plantation and do what he wanted, with minimal privacy if any. It was common for a "house" female (housekeeper, maid, cook, laundress, or nanny) to be raped by one or more members of the household. Houses of prostitution throughout the slave states were largely staffed by female slaves providing sexual services, to their owners' profit. There were a small number of free black females engaged in prostitution, or concubinage, especially in New Orleans.

Slave owners who engaged in sexual activity with female slaves "were often the elite of the community. They had little need to worry about public scorn." These relationships "appear to have been tolerated and in some cases even quietly accepted". "Southern women ... do not trouble themselves about it". Franklin and Armfield, who were definitely the elite of the community, joked frequently in their letters about the black women and girls that they were raping. It never occurred to them that there was anything wrong in what they were doing.

Light-skinned young girls were sold openly for sexual use; their price was much higher than that of a field hand. Special markets for the fancy girl trade existed in New Orleans and Lexington, Kentucky. Historian Philip Shaw describes an occasion when Abraham Lincoln and Allen Gentry witnessed such sales in New Orleans in 1828:

Gentry vividly remembered a day in New Orleans when he and the nineteen-year-old Lincoln came upon a slave market. Pausing to watch, Gentry recalled looking down at Lincoln's hands and seeing that he "doubled his fists tightly; his knuckles went white". Men wearing black coats and white hats buy field hands, "black and ugly", for $500 to 800. And then the real horror begins: "When the sale of "fancy girls" began, Lincoln, "unable to stand it any longer", muttered to Gentry "Allen that's a disgrace. If I ever get a lick at that thing I'll hit it hard."

Those girls who were "considered educated and refined, were purchased by the wealthiest clients, usually plantation owners, to become personal sexual companions". "There was a great demand in New Orleans for 'fancy girls'."

The issue that did come up frequently was the threat of sexual intercourse between black males and white females. Just as the black women were perceived as having "a trace of Africa, that supposedly incited passion and sexual wantonness", the men were perceived as savages, unable to control their lust, given an opportunity.

Another approach to the question was offered by Quaker and Florida planter Zephaniah Kingsley, Jr. He advocated, and personally practiced, deliberate racial mixing through marriage, as part of his proposed solution to the slavery issue: racial integration, called "amalgamation" at the time. In an 1829 Treatise, he stated that mixed-race people were healthier and often more beautiful, that interracial sex was hygienic, and slavery made it convenient. Because of these views, tolerated in Spanish Florida, he found it impossible to remain long in Territorial Florida, and moved with his slaves and multiple wives to a plantation, Mayorasgo de Koka, in Haiti (now in the Dominican Republic). There were many others who less flagrantly practiced interracial, common-law marriages with slaves (see Partus sequitur ventrem).

To help regulate the relationship between slave and owner, including legal support for keeping the slave as property, states established slave codes, most based on laws existing since the colonial era. The code for the District of Columbia defined a slave as "a human being, who is by law deprived of his or her liberty for life, and is the property of another".

While each state had its own slave code, many concepts were shared throughout the slave states. According to the slave codes, some of which were passed in reaction to slave rebellions, teaching a slave to read or write was illegal. This prohibition was unique to American slavery, believed to reduce slaves forming aspirations that could lead to escape or rebellion. Informal education occurred when white children taught slave companions what they were learning; in other cases, adult slaves learned from free artisan workers, especially if located in cities, where there was more freedom of movement.

In Alabama, slaves were not allowed to leave their master's premises without written consent or passes. This was a common requirement in other states as well, and locally run patrols (known to slaves as pater rollers) often checked the passes of slaves who appeared to be away from their plantations. In Alabama slaves were prohibited from trading goods among themselves. In Virginia, a slave was not permitted to drink in public within one mile of his master or during public gatherings. Slaves were not permitted to carry firearms in any of the slave states.

Slaves were generally prohibited by law from associating in groups, with the exception of worship services (a reason why the Black Church is such a notable institution in black communities today). Following Nat Turner's rebellion in 1831, which raised white fears throughout the South, some states also prohibited or restricted religious gatherings of slaves, or required that they be officiated by white men. Planters feared that group meetings would facilitate communication among slaves that could lead to rebellion. Slaves held private, secret "brush meetings" in the woods.

In Ohio, an emancipated slave was prohibited from returning to the state in which he or she had been enslaved. Other Northern states discouraged the settling of free blacks within their boundaries. Fearing the influence of free blacks, Virginia and other Southern states passed laws to require blacks who had been freed to leave the state within a year (or sometimes less time) unless granted a stay by an act of the legislature.

Africans brought their religions with them from Africa, including Islam, Catholicism, and traditional religions.

Prior to the American Revolution, masters and revivalists spread Christianity to slave communities, including Catholicism in Spanish Florida and California, and in French and Spanish Louisiana, and Protestantism in English colonies, supported by the Society for the Propagation of the Gospel. In the First Great Awakening of the mid-18th century, Baptists and Methodists from New England preached a message against slavery, encouraged masters to free their slaves, converted both slaves and free blacks, and gave them active roles in new congregations. The first independent black congregations were started in the South before the Revolution, in South Carolina and Georgia. Believing that, "slavery was contrary to the ethics of Jesus", Christian congregations and church clergy, especially in the North, played a role in the Underground Railroad, especially Wesleyan Methodists, Quakers and Congregationalists.

Over the decades and with the growth of slavery throughout the South, some Baptist and Methodist ministers gradually changed their messages to accommodate the institution. After 1830, white Southerners argued for the compatibility of Christianity and slavery, with a multitude of both Old and New Testament citations. They promoted Christianity as encouraging better treatment of slaves and argued for a paternalistic approach. In the 1840s and 1850s, the issue of accepting slavery split the nation's largest religious denominations (the Methodist, Baptist and Presbyterian churches) into separate Northern and Southern organizations (see Methodist Episcopal Church, South, Southern Baptist Convention, and Presbyterian Church in the Confederate States of America). Schisms occurred, such as that between the Wesleyan Methodist Church and the Methodist Episcopal Church.

Southern slaves generally attended their masters' white churches, where they often outnumbered the white congregants. They were usually permitted to sit only in the back or in the balcony. They listened to white preachers, who emphasized the obligation of slaves to keep in their place, and acknowledged the slave's identity as both person and property. Preachers taught the master's responsibility and the concept of appropriate paternal treatment, using Christianity to improve conditions for slaves, and to treat them "justly and fairly" (Col. 4:1). This included masters having self-control, not disciplining under anger, not threatening, and ultimately fostering Christianity among their slaves by example.

Slaves also created their own religious observances, meeting alone without the supervision of their white masters or ministers. The larger plantations with groups of slaves numbering 20, or more, tended to be centers of nighttime meetings of one or several plantation slave populations. These congregations revolved around a singular preacher, often illiterate with limited knowledge of theology, who was marked by his personal piety and ability to foster a spiritual environment. African Americans developed a theology related to Biblical stories having the most meaning for them, including the hope for deliverance from slavery by their own Exodus. One lasting influence of these secret congregations is the African American spiritual.

In a feature unique to American slavery, legislatures across the South enacted new laws to curtail the already limited rights of African Americans. For example, Virginia prohibited blacks, free or slave, from practicing preaching, prohibited them from owning firearms, and forbade anyone to teach slaves or free blacks how to read. It specified heavy penalties for both student and teacher if slaves were taught, including whippings or jail.

[E]very assemblage of negroes for the purpose of instruction in reading or writing, or in the night time for any purpose, shall be an unlawful assembly. Any justice may issue his warrant to any office or other person, requiring him to enter any place where such assemblage may be, and seize any negro therein; and he, or any other justice, may order such negro to be punished with stripes.

Slave owners saw literacy as a threat to the institution of slavery and their financial investment in it; as a North Carolina statute passed in 1830-1831 stated, "Teaching slaves to read and write, tends to excite dissatisfaction in their minds, and to produce insurrection and rebellion." Literacy enabled the enslaved to read the writings of abolitionists, which discussed the abolition of slavery and described the slave revolution in Haiti of 1791–1804 and the end of slavery in the British Empire in 1833. It also allowed slaves to learn that thousands of enslaved individuals had escaped, often with the assistance of the Underground Railroad. Literacy also was believed to make the enslaved unhappy at best, insolent and sullen at worst. As put by prominent Washington lawyer Elias B. Caldwell in 1822:

The more you improve the condition of these people, the more you cultivate their minds, the more miserable you make them, in their present state. You give them a higher relish for those privilegies which they can never attain, and turn what we intend for a blessing [slavery] into a curse. No, if they must remain in their present situation, keep them in the lowest state of degradation and ignorance. The nearer you bring them to the condition of brutes, the better chance do you give them of possessing their apathy.

Unlike in the South, slave owners in Utah were required to send their slaves to school. Black slaves did not have to spend as much time in school as Indian slaves.

With the development of slave and free states after the American Revolution, and far-flung commercial and military activities, new situations arose in which slaves might be taken by masters into free states. Most free states not only prohibited slavery, but ruled that slaves brought and kept there illegally could be freed. Such cases were sometimes known as transit cases. Dred Scott and his wife Harriet Scott each sued for freedom in St. Louis after the death of their master, based on their having been held in a free territory (the northern part of the Louisiana Purchase from which slavery was excluded under the terms of the Missouri Compromise). (Later the two cases were combined under Dred Scott's name.) Scott filed suit for freedom in 1846 and went through two state trials, the first denying and the second granting freedom to the couple (and, by extension, their two daughters, who had also been held illegally in free territories). For 28 years, Missouri state precedent had generally respected laws of neighboring free states and territories, ruling for freedom in such transit cases where slaves had been held illegally in free territory. But in the Dred Scott case, the Missouri Supreme Court ruled against the slaves.

After Scott and his team appealed the case to the U.S. Supreme Court, Chief Justice Roger B. Taney, in a sweeping decision, denied Scott his freedom. The 1857 decision, decided 7–2, held that a slave did not become free when taken into a free state; Congress could not bar slavery from a territory; and people of African descent imported into the United States and held as slaves, or their descendants, could never be citizens and thus had no status to bring suit in a U.S. court. A state could not bar slaveowners from bringing slaves into that state. Many Republicans, including Abraham Lincoln, considered the decision unjust and evidence that the Slave Power had seized control of the Supreme Court. Anti-slavery groups were enraged and slave owners encouraged, escalating the tensions that led to civil war.

In 1850, Congress passed the Fugitive Slave Act, as part of the Compromise of 1850, which required law enforcement and citizens of free states to cooperate in the capture and return of slaves. This met with considerable overt and covert resistance in free states and cities such as Philadelphia, New York, and Boston. Refugees from slavery continued to flee the South across the Ohio River and other parts of the Mason–Dixon line dividing North from South, to the North and Canada via the Underground Railroad. Some white Northerners helped hide former slaves from their former owners or helped them reach freedom in Canada.

As part of the Compromise of 1850, Congress abolished the slave trade (though not the ownership of slaves) in the District of Columbia; fearing this would happen, Alexandria, regional slave trading center and port, successfully sought its removal from the District of Columbia and devolution to Virginia. After 1854, Republicans argued that the "Slave Power", especially the pro-slavery Democratic Party in the South, controlled two of the three branches of the Federal government.

The abolitionists, realizing that the total elimination of slavery was unrealistic as an immediate goal, worked to prevent the expansion of slavery into the western territories that eventually would become new states. The Missouri Compromise, the Compromise of 1850, and the Bleeding Kansas period dealt with whether new states would be slave or free, or how that was to be decided. Both sides were anxious about effects of these decisions on the balance of power in the Senate.

After the passage of the Kansas–Nebraska Act in 1854, border fighting broke out in the Kansas Territory, where the question of whether it would be admitted to the Union as a slave or free state was left to the inhabitants. Migrants from both free and slave states moved into the territory to prepare for the vote on slavery. Abolitionist John Brown, the most famous of the anti-slavery immigrants, was active in the fighting in "Bleeding Kansas", but so too were many white Southerners (many from adjacent Missouri) who opposed abolition.

Abraham Lincoln's and the Republicans' political platform in 1860 was to stop slavery's expansion. Historian James M. McPherson says that in his famous "House Divided" speech in 1858, Lincoln said American republicanism can be purified by restricting the further expansion of slavery as the first step to putting it on the road to 'ultimate extinction.' Southerners took Lincoln at his word. When he won the presidency, they left the Union to escape the 'ultimate extinction' of slavery."

The divisions became fully exposed with the 1860 presidential election. The electorate split four ways. The Southern Democrats endorsed slavery, while the Republican Party denounced it. The Northern Democrats said democracy required the people to decide on slavery locally, state by state and territory by territory. The Constitutional Union Party said the survival of the Union was at stake and everything else should be compromised.

Lincoln, the Republican, won with a plurality of popular votes and a majority of electoral votes. Lincoln, however, did not appear on the ballots of ten southern slave states. Many slave owners in the South feared that the real intent of the Republicans was the abolition of slavery in states where it already existed, and that the sudden emancipation of four million slaves would be disastrous for the slave owners and for the economy that drew its greatest profits from the labor of people who were not paid. The slave owners feared that ending the balance could lead to the domination of the federal government by the northern free states. This led seven southern states to secede from the Union. When the Confederate Army attacked a U.S. Army installation at Fort Sumter, the American Civil War began and four additional slave states seceded. Northern leaders had viewed the slavery interests as a threat politically, but with secession, they viewed the prospect of a new Southern nation, the Confederate States of America, with control over the Mississippi River and parts of the West, as politically unacceptable. Most of all, they could not accept this repudiation of American nationalism.

The American Civil War, beginning in 1861, led to the end of chattel slavery in America. Not long after the war broke out, through a legal maneuver by Union General Benjamin F. Butler, a lawyer by profession, slaves who fled to Union lines were considered "contraband of war". General Butler ruled that they were not subject to return to Confederate owners as they had been before the war. "Lincoln and his Cabinet discussed the issue on May 30 and decided to support Butler's stance". Soon word spread, and many slaves sought refuge in Union territory, desiring to be declared "contraband". Many of the "contrabands" joined the Union Army as workers or troops, forming entire regiments of the U.S. Colored Troops. Others went to refugee camps such as the Grand Contraband Camp near Fort Monroe or fled to northern cities. General Butler's interpretation was reinforced when Congress passed the Confiscation Act of 1861, which declared that any property used by the Confederate military, including slaves, could be confiscated by Union forces.

At the beginning of the war, some Union commanders thought they were supposed to return escaped slaves to their masters. By 1862, when it became clear that this would be a long war, the question of what to do about slavery became more general. The Southern economy and military effort depended on slave labor. It began to seem unreasonable to protect slavery while blockading Southern commerce and destroying Southern production. As Congressman George W. Julian of Indiana put it in an 1862 speech in Congress, the slaves "cannot be neutral. As laborers, if not as soldiers, they will be allies of the rebels, or of the Union." Julian and his fellow Radical Republicans put pressure on Lincoln to rapidly emancipate the slaves, whereas moderate Republicans favored gradual, compensated emancipation and voluntary colonization. The border states, Peace Democrats (Copperheads), and War Democrats opposed emancipation, although the border states and War Democrats eventually accepted it as part of the total war needed to save the Union.

The Emancipation Proclamation was an executive order issued by President Abraham Lincoln on January 1, 1863. In a single stroke it changed the legal status of three million slaves in designated areas of the Confederacy from "slave" to "free". It had the practical effect that as soon as a slave escaped the control of his or her owner, by running away or through advances of federal troops, the slave's proclaimed freedom became actual. Plantation owners, realizing that emancipation would destroy their economic system, sometimes moved their slaves as far as possible out of reach of the Union army. By June 1865, the Union Army controlled all of the Confederacy and had liberated all of the designated slaves.

In 1861, Lincoln expressed the fear that premature attempts at emancipation would mean the loss of the border states. He believed that "to lose Kentucky is nearly the same as to lose the whole game." At first, Lincoln reversed attempts at emancipation by Secretary of War Simon Cameron and Generals John C. Frémont (in Missouri) and David Hunter (in South Carolina, Georgia and Florida) to keep the loyalty of the border states and the War Democrats.

On July 22, 1862, Lincoln told his cabinet of his plan to issue a preliminary Emancipation Proclamation. Secretary of State William H. Seward advised Lincoln to wait for a victory before issuing the proclamation, as to do otherwise would seem like "our last shriek on the retreat". On September 17, 1862, the Battle of Antietam provided this opportunity, and on September 22, 1862, Lincoln issued his preliminary Emancipation Proclamation, which provided that enslaved people in the states in rebellion against the United States on January 1, 1863, "shall be then, thenceforward, and forever free". On September 24 and 25, the War Governors' Conference added support for the proclamation. Lincoln issued his final Emancipation Proclamation on January 1, 1863. In his letter to Albert G. Hodges, Lincoln explained his belief that

If slavery is not wrong, nothing is wrong ... And yet I have never understood that the Presidency conferred upon me an unrestricted right to act officially upon this judgment and feeling ... I claim not to have controlled events, but confess plainly that events have controlled me.

Lincoln's Emancipation Proclamation declared freedom for slaves in the Confederate states and authorized the enlistment of African Americans in the Union Army. The Emancipation Proclamation did not free slaves in the border states, which were the slaveholding states that remained in the Union. As a practical matter, the proclamation freed only those slaves who escaped to Union lines. But the proclamation made the abolition of slavery an official war goal and was implemented as the Union took territory from the Confederacy. According to the Census of 1860, this policy would free nearly four million slaves, or over 12 percent of the total population of the United States.

Because the Emancipation Proclamation was issued under the president's war powers, it might not have continued in force after the war ended. Therefore, Lincoln played a leading role in getting the constitutionally required two-thirds majority of both houses of Congress to vote for the Thirteenth Amendment, which made emancipation universal and permanent, "except as a punishment for crime".

Enslaved African Americans had not waited for Lincoln before escaping and seeking freedom behind Union lines. From the early years of the war, hundreds of thousands of African Americans escaped to Union lines, especially in Union-controlled areas such as Norfolk and the Hampton Roads region in 1862 Virginia, Tennessee from 1862 on, and the line of Sherman's march. So many African Americans fled to Union lines that commanders created camps and schools for them, where both adults and children learned to read and write. The American Missionary Association entered the war effort by sending teachers south to such contraband camps, for instance, establishing schools in Norfolk and on nearby plantations.

In addition, nearly 200,000 African-American men served with distinction in the Union forces as soldiers and sailors; most were escaped slaves. The Confederacy was outraged by armed black soldiers and refused to treat them as prisoners of war. They murdered many, as at the Fort Pillow massacre, and re-enslaved others.

On February 24, 1863, the Arizona Organic Act abolished slavery in the newly formed Arizona Territory. Tennessee and all of the border states (except Kentucky and Delaware) abolished slavery by early 1865. Thousands of slaves were freed by the operation of the Emancipation Proclamation as Union armies marched across the South. Emancipation came to the remaining Southern slaves after the surrender of all the Confederate troops in spring 1865.

In spite of the South's shortage of manpower, until 1865, most Southern leaders opposed arming slaves as soldiers. However, a few Confederates discussed arming slaves. Finally, in early 1865, General Robert E. Lee said that black soldiers were essential, and legislation was passed. The first black units were in training when the war ended in April.

Booker T. Washington remembered Emancipation Day in early 1863, when he was a boy of nine in Virginia:

As the great day drew nearer, there was more singing in the slave quarters than usual. It was bolder, had more ring, and lasted later into the night. Most of the verses of the plantation songs had some reference to freedom. ... Some man who seemed to be a stranger (a United States officer, I presume) made a little speech and then read a rather long paper – the Emancipation Proclamation, I think. After the reading we were told that we were all free, and could go when and where we pleased. My mother, who was standing by my side, leaned over and kissed her children, while tears of joy ran down her cheeks. She explained to us what it all meant, that this was the day for which she had been so long praying, but fearing that she would never live to see.

The war ended on June 22, 1865, and following that surrender, the Emancipation Proclamation was enforced throughout remaining regions of the South that had not yet freed the slaves. Slavery officially continued for a couple of months in other locations. Federal troops arrived in Galveston, Texas, on June 19, 1865, to enforce the emancipation. The commemoration of that event, Juneteenth National Independence Day, was declared a national holiday in 2021.

The Thirteenth Amendment, abolishing slavery except as punishment for a crime, had been passed by the Senate in April 1864, and by the House of Representatives in January 1865.

The amendment did not take effect until it was ratified by three-fourths of the states, which occurred on December 6, 1865, when Georgia ratified it. On that date, the last 40,000–45,000 enslaved Americans in the remaining two slave states of Kentucky and Delaware, as well as the 200 or so perpetual apprentices in New Jersey left from the very gradual emancipation process begun in 1804, were freed. The last Americans known to have been born into legal slavery died in the 1970s.

Journalist Douglas A. Blackmon reported in his Pulitzer Prize-winning book Slavery By Another Name that many black persons were virtually enslaved under convict leasing programs, which started after the Civil War. Most Southern states had no prisons; they leased convicts to businesses and farms for their labor, and the lessee paid for food and board. Incentives for abuse were present.

The continued involuntary servitude took various forms, but the primary forms included convict leasing, peonage and sharecropping, with the latter eventually encompassing poor whites as well. By the 1930s, whites constituted most of the sharecroppers in the South. Mechanization of agriculture had reduced the need for farm labor, and many black people left the South in the Great Migration. Jurisdictions and states created fines and sentences for a wide variety of minor crimes and used these as an excuse to arrest and sentence black people. Under convict-leasing programs, African-American men, often guilty of petty crimes or even no crime at all, were arrested, compelled to work without pay, repeatedly bought and sold, and coerced to do the bidding of the leaseholder. Sharecropping, as it was practiced during this period, often involved severe restrictions on the freedom of movement of sharecroppers, who could be whipped for leaving the plantation. Both sharecropping and convict leasing were legal and tolerated by both the North and South. However, peonage was an illicit form of forced labor. Its existence was ignored by authorities while thousands of African Americans and poor white Americans were subjugated and held in bondage until the mid-1960s to the late 1970s. With the exception of cases of peonage, beyond the period of Reconstruction, the federal government took almost no action to enforce the Thirteenth Amendment until December 1941, when President Franklin Delano Roosevelt summoned his attorney general. Five days after the attack on Pearl Harbor, at the request of the President, Attorney General Francis Biddle issued Circular No. 3591 to all federal prosecutors, instructing them to investigate actively and try any case of involuntary servitude or slavery. Several months later, convict leasing was officially abolished. But aspects have persisted in other forms. Historians argue that other systems of penal labor were all created in 1865, and convict leasing was simply the most oppressive form. Over time, a large civil rights movement arose to bring full civil rights and equality under the law to all Americans.

With emancipation a legal reality, white Southerners were concerned with both controlling the newly freed slaves and keeping them in the labor force at the lowest level. The system of convict leasing began during Reconstruction and was fully implemented in the 1880s, officially ending in the last state, Alabama, in 1928. It persisted in various forms until it was abolished in 1942 by President Franklin D. Roosevelt during World War II, several months after the attack on Pearl Harbor involved the U.S. in the conflict. This system allowed private contractors to purchase the services of convicts from the state or local governments for a specific time period. African Americans, due to "vigorous and selective enforcement of laws and discriminatory sentencing", made up the vast majority of the convicts leased. Writer Douglas A. Blackmon writes of the system:

It was a form of bondage distinctly different from that of the antebellum South in that for most men, and the relatively few women drawn in, this slavery did not last a lifetime and did not automatically extend from one generation to the next. But it was nonetheless slavery – a system in which armies of free men, guilty of no crimes and entitled by law to freedom, were compelled to labor without compensation, were repeatedly bought and sold, and were forced to do the bidding of white masters through the regular application of extraordinary physical coercion.

The constitutional basis for convict leasing is that the Thirteenth Amendment, while abolishing slavery and involuntary servitude generally, expressly permits it as a punishment for crime.

Historian Mark Summers Wahlgren notes that the estimated literacy rate among formerly enslaved southern blacks at the time of emancipation was five to 10 percent, but had reached a baseline of 40 to 50 percent (and higher in cities) by the turn of the century, representing a "great advance". As W. E. B. Du Bois noted, the black colleges were not perfect, but "in a single generation they put thirty thousand black teachers in the South" and "wiped out the illiteracy of the majority of black people in the land".

Northern philanthropists continued to support black education in the 20th century, for example of a major donor to Hampton Institute and Tuskegee was George Eastman, who also helped fund health programs at colleges and in communities.

In the 21st century, various legislative bodies have issued public apologies for slavery in the United States.

A 2016 study, published in The Journal of Politics, finds that "[w]hites who currently live in Southern counties that had high shares of slaves in 1860 are more likely to identify as a Republican, oppose affirmative action, and express racial resentment and colder feelings toward blacks." The study contends that "contemporary differences in political attitudes across counties in the American South in part trace their origins to slavery's prevalence more than 150 years ago. " The authors argue that their findings are consistent with the theory that "following the Civil War, Southern whites faced political and economic incentives to reinforce existing racist norms and institutions to maintain control over the newly freed African American population. This amplified local differences in racially conservative political attitudes, which in turn have been passed down locally across generations."

A 2017 study in the British Journal of Political Science argued that the British American colonies without slavery adopted better democratic institutions to attract migrant workers to their colonies.

An article published in the Journal of Economic History in 2022 finds that former slave owners remained politically dominant long after the abolition of slavery. Using data from Texas, the authors find that "[i]n 1900, still around 50 percent of all state legislators came from a slave-owning background."

Robert Fogel and Stanley Engerman, in their 1974 book Time on the Cross, argued that the rate of return of slavery at the market price was close to ten percent, a number close to investment in other assets. The transition from indentured servants to slaves is cited to show that slaves offered greater profits to their owners. A qualified consensus among economic historians and economists is that "Slave agriculture was efficient compared with free agriculture. Economies of scale, effective management, and intensive utilization of labor and capital made southern slave agriculture considerably more efficient than nonslave southern farming", and it is the near-universal consensus among economic historians and economists that slavery was not "a system irrationally kept in existence by plantation owners who failed to perceive or were indifferent to their best economic interests".

The relative price of slaves and indentured servants in the antebellum period did decrease. Indentured servants became more costly with the increase in the demand of skilled labor in England. At the same time, slaves were mostly supplied from within the United States and thus language was not a barrier, and the cost of transporting slaves from one state to another was relatively low. However, as in Brazil and Europe, slavery at its end in the United States tended to be concentrated in the poorest regions of the United States, with a qualified consensus among economists and economic historians concluding that the "modern period of the South's economic convergence to the level of the North only began in earnest when the institutional foundations of the southern regional labor market were undermined, largely by federal farm and labor legislation dating from the 1930s."

In the decades preceding the Civil War, the black population of the United States experienced a rapid natural increase. Unlike the trans-Saharan slave trade with Africa, the slave population transported by the Atlantic slave trade to the United States was sex-balanced. The slave population multiplied nearly fourfold between 1810 and 1860, despite the passage of the Act Prohibiting Importation of Slaves signed into law by President Thomas Jefferson in 1807 banning the international slave trade. Thus, it is also the universal consensus among modern economic historians and economists that slavery in the United States was not "economically moribund on the eve of the Civil War". In the 2010s, several historians, among them Edward E. Baptist, Sven Beckert, Walter Johnson and Calvin Schermerhorn, have posited that slavery was integral in the development of American capitalism. Johnson wrote in River of Dark Dreams (2013): "The cords of credit and debt—of advance and obligation—that cinched the Atlantic economy together were anchored with the mutually defining values of land and slaves: without land and slaves, there was no credit, and without slaves, land itself was valueless. Promises made in the Mississippi Valley were backed by the value of slaves and fulfilled in their labor." Other economic historians have rejected that thesis.

A 2023 study estimates that prior to the onset of the US Civil War, the enslaved population produced 12.6% of US national product.

Slavery had a long-lasting impact on wealth and racial inequality in the United States. Black families whose ancestors were freed before the start of the Civil War have substantially better socio-economic outcomes than families who were freed in the Civil War.

Scholars disagree on how to quantify the efficiency of slavery. In Time on the Cross Fogel and Engerman equate efficiency to total factor productivity (TFP), the output per average unit of input on a farm. Using this measurement, Southern farms that enslaved black people using the gang system were 35% more efficient than Northern farms, which used free labor. Under the gang system, groups of slaves perform synchronized tasks under the constant vigilance of an overseer. Each group was like a part of a machine. If perceived to be working below his capacity, a slave could be punished. Fogel argues that this kind of negative enforcement was not frequent and that slaves and free laborers had a similar quality of life; however, there is controversy on this last point. A critique of Fogel and Engerman's view was published by Paul A. David in 1976.

In 1995, a random survey of 178 members of the Economic History Association sought to study the views of economists and economic historians on the debate. The study found that 72 percent of economists and 65 percent of economic historians would generally agree that "Slave agriculture was efficient compared with free agriculture. Economies of scale, effective management, and intensive utilization of labor and capital made southern slave agriculture considerably more efficient than nonslave southern farming." 48 percent of the economists agreed without provisos, while 24 percent agreed when provisos were included in the statement. On the other hand, 58 percent of economic historians and 42 percent of economists disagreed with Fogel and Engerman's "proposition that the material (not psychological) conditions of the lives of slaves compared favorably with those of free industrial workers in the decades before the Civil War".

The U.S. has a capitalist economy so the price of slaves was determined by the law of supply and demand. For example, following bans on the import of slaves after the UK's Slave Trade Act 1807 and the American 1807 Act Prohibiting Importation of Slaves, the prices for slaves increased. The markets for the products produced by slaves also affected the price of slaves (e.g. the price of slaves fell when the price of cotton fell in 1840). Anticipation of slavery's abolition also influenced prices. During the Civil War the price for slave men in New Orleans dropped from $1,381 in 1861 to $1,116 by 1862 (the city was captured by U.S. forces in the Spring of 1862).

Controlling for inflation, prices of slaves rose dramatically in the six decades prior to the Civil War, reflecting demand due to commodity cotton, as well as use of slaves in shipping and manufacturing. Although the prices of slaves relative to indentured servants declined, both got more expensive. Cotton production was rising and relied on the use of slaves to yield high profits. Fogel and Engeman initially argued that if the Civil War had not happened, the slave prices would have increased even more, an average of more than fifty percent by 1890.

Prices reflected the characteristics of the slave; such factors as sex, age, nature, and height were all taken into account to determine the price of a slave. Over the life-cycle, the price of enslaved women was higher than their male counterparts up to puberty age, as they would likely bear children who their masters could sell as slaves and could be used as slave laborers. Men around the age of 25 were the most valued, as they were at the highest level of productivity and still had a considerable life-span. If slaves had a history of fights or escapes, their price was lowered reflecting what planters believed was risk of repeating such behavior. Slave traders and buyers would examine a slave's back for whipping scars; a large number of injuries would be seen as evidence of laziness or rebelliousness, rather than the previous master's brutality, and would lower the slave's price. Taller male slaves were priced at a higher level, as height was viewed as a proxy for fitness and productivity.

While slavery brought profits in the short run, discussion continues on the economic benefits of slavery in the long run. In 1995, a random anonymous survey of 178 members of the Economic History Association found that out of the forty propositions about American economic history that were surveyed, the group of propositions most disputed by economic historians and economists were those about the postbellum economy of the American South (along with the Great Depression). The only exception was the proposition initially put forward by historian Gavin Wright that the "modern period of the South's economic convergence to the level of the North only began in earnest when the institutional foundations of the southern regional labor market were undermined, largely by federal farm and labor legislation dating from the 1930s." 62 percent of economists (24 percent with and 38 percent without provisos) and 73 percent of historians (23 percent with and 50 percent without provisos) agreed with this statement. Wright has also argued that the private investment of monetary resources in the cotton industry, among others, delayed development in the South of commercial and industrial institutions. There was little public investment in railroads or other infrastructure. Wright argues that agricultural technology was far more developed in the South, representing an economic advantage of the South over the North of the United States.

In Democracy in America, Alexis de Tocqueville noted that "the colonies in which there were no slaves became more populous and more rich than those in which slavery flourished". In 1857, in The Impending Crisis of the South: How to Meet It, Hinton Rowan Helper made the same point. Economists Peter H. Lindert and Jeffrey G. Williamson, in a pair of articles published in 2012 and 2013, found that, despite the American South initially having per capita income roughly double that of the North in 1774, incomes in the South had declined 27% by 1800 and continued to decline over the next four decades, while the economies in New England and the Mid-Atlantic states vastly expanded. By 1840, per capita income in the South was well behind the Northeast and the national average (Note: this is also true in the early 21st century).

Lindert and Williamson argue that this antebellum period is an example of what economists Daron Acemoglu, Simon Johnson, and James A. Robinson call "a reversal of fortune". In his essay "The Real History of Slavery", economist Thomas Sowell reiterated and augmented the observation made by de Tocqueville by comparing slavery in the United States to slavery in Brazil. He notes that slave societies reflected similar economic trends in those and other parts of the world, suggesting that the trend Lindert and Williamson identify may have continued until the American Civil War:

Both in Brazil and in the United States – the countries with the two largest slave populations in the Western Hemisphere – the end of slavery found the regions in which slaves had been concentrated poorer than other regions of these same countries. For the United States, a case could be made that this was due to the Civil War, which did so much damage to the South, but no such explanation would apply to Brazil, which fought no Civil War over this issue. Moreover, even in the United States, the South lagged behind the North in many ways even before the Civil War. Although slavery in Europe died out before it was abolished in the Western Hemisphere, as late as 1776 slavery had not yet died out all across the continent when Adam Smith wrote in The Wealth of Nations that it still existed in some eastern regions. But, even then, Eastern Europe was much poorer than Western Europe. The slavery of North Africa and the Middle East, over the centuries, took more slaves from sub-Saharan Africa than the Western Hemisphere did ... But these remained largely poor countries until the discovery and extraction of their vast oil deposits.

Sowell also notes in Ethnic America: A History, citing historians Clement Eaton and Eugene Genovese, that three-quarters of Southern white families owned no slaves at all. Most slaveholders lived on farms rather than plantations, and few plantations were as large as the fictional ones depicted in Gone with the Wind. In "The Real History of Slavery", Sowell also notes in comparison to slavery in the Arab world and the Middle East (where slaves were seldom used for productive purposes) and China (where the slaves consumed the entire output they created), Sowell observes that many commercial slaveowners in the antebellum South tended to be spendthrift and many lost their plantations due to creditor foreclosures, and in Britain, profits by British slave traders only amounted to two percent of British domestic investment at the height of the Atlantic slave trade in the 18th century. Sowell draws the following conclusion regarding the macroeconomic value of slavery:

In short, even though some individual slaveowners grew rich and some family fortunes were founded on the exploitation of slaves, that is very different from saying that the whole society, or even its non-slave population as a whole, was more economically advanced than it would have been in the absence of slavery. What this means is that, whether employed as domestic servants or producing crops or other goods, millions suffered exploitation and dehumanization for no higher purpose than the ... aggrandizement of slaveowners.

Eric Hilt noted that, while some historians have suggested slavery was necessary for the Industrial Revolution (on the grounds that American slave plantations produced most of the raw cotton for the British textiles market and the British textiles market was the vanguard of the Industrial Revolution), it is not clear if this is actually true; there is no evidence that cotton could not have been mass-produced by yeoman farmers rather than slave plantations if the latter had not existed (as their existence tended to force yeoman farmers into subsistence farming) and there is some evidence that they certainly could have. The soil and climate of the American South were excellent for growing cotton, so it is not unreasonable to postulate that farms without slaves could have produced substantial amounts of cotton; even if they did not produce as much as the plantations did, it could still have been enough to serve the demand of British producers. Similar arguments have been made by other historians.

Scholar Adrienne Davis articulates how the economics of slavery also can be defined as a sexual economy, specifically focusing on how black women were expected to perform physical, sexual and reproductive labor to provide a consistent enslaved workforce and increase the profits of white slavers. Davis writes that black women were needed for their "sexual and reproductive labor to satisfy the economic, political, and personal interest of white men of the elite class" articulating that black women's reproductive capacity was important in the maintenance of the system of slavery due to its ability to perpetuate an enslaved workforce. She is also drawing attention to black women's labor being needed to maintain the aristocracy of a white ruling class, due to the intimate nature of reproduction and its potential for producing more enslaved peoples.

Due to the institution of partus sequitur ventrem, black women's wombs became the site where slavery was developed and transferred, meaning that black women were not only used for their physical labor, but for their sexual and reproductive labor as well.

"The rule that the children's status follows their mothers' was a foundational one for our economy. It converted enslaved women's reproductive capacity into market capital"

This articulation by Davis illustrates how black women's reproductive capacity was commodified under slavery, and that an analysis of the economic structures of slavery requires an acknowledgment of how pivotal black women's sexuality was in maintaining slavery's economic power. Davis writes how black women performed labor under slavery, writing: "[black women were] male when convenient and horrifically female when needed". The fluctuating expectations of black women's gendered labor under slavery disrupted the white normative roles that were assigned to white men and white women. This ungendering black women received under slavery contributed to the systemic dehumanization experienced by enslaved black women, as they were unable to receive the expectations or experiences of either gender within the white binary.

Davis's arguments address the fact that, under slavery, black women's sexuality became linked to the economic and public sphere, making their intimate lives into public institutions. Black women's physical labor was gendered as masculine under slavery when they were needed to yield more profit, but their reproductive capacities and sexual labor were equally as important in maintaining white power over black communities and perpetuating an enslaved workforce.

About 600,000 slaves were transported to the United States, or five percent of the 12 million slaves taken from Africa. About 310,000 of these persons were imported into the Thirteen Colonies before 1776: 40 percent directly, and the rest from the Caribbean.

Slaves trafficked to the British colonies and United States:

Time period	Quantity
1620–1700	21,000
1701–1760	189,000
1761–1770	63,000
1771–1790	56,000
1791–1800	79,000
1801–1810	124,000
1810–1865	51,000
Total	597,000

The great majority of enslaved Africans were transported to sugar plantations in the Caribbean and to Portuguese Brazil. As life expectancy was short, their numbers had to be continually replenished. Life expectancy was much higher in the United States, and the enslaved population was successful in reproduction, which was called "natural increase" by enslavers. The population of enslaved people in the United States grew to 4 million by the 1860 census. Historian J. David Hacker conducted research that estimated that the cumulative number of slaves in colonial America and the United States (1619–1865) was 10 million.

Origins and percentages of Africans imported into British North America and Louisiana (1700–1820)	Amount % (exceeds 100%)
West-central Africa (Kongo, N. Mbundu, S. Mbundu)	26.1
Bight of Biafra (Igbo, Tikar, Ibibio, Bamileke, Bubi)	24.4
Sierra Leone (Mende, Temne)	15.8
Senegambia (Mandinka, Fula, Wolof)	14.5
Gold Coast (Akan, Fon)	13.1
Windward Coast (Mandé, Kru)	5.2
Bight of Benin (Yoruba, Ewe, Fon, Allada and Mahi)	4.3
Southeast Africa (Macua, Malagasy)	1.8

Census Year	# Slaves	# Free Africans	Total Africans	% Free Africans	Total US population	% Africans of total
1790	697,681	59,527	757,208	8%	3,929,214	19%
1800	893,602	108,435	1,002,037	11%	5,308,483	19%
1810	1,191,362	186,446	1,377,808	14%	7,239,881	19%
1820	1,538,022	233,634	1,771,656	13%	9,638,453	18%
1830	2,009,043	319,599	2,328,642	14%	12,860,702	18%
1840	2,487,355	386,293	2,873,648	13%	17,063,353	17%
1850	3,204,313	434,495	3,638,808	12%	23,191,876	16%
1860	3,953,760	488,070	4,441,830	11%	31,443,321	14%
1870	0	4,880,009	4,880,009	100%	38,558,371	13%
Source:"Distribution of Slaves in U.S. History". Retrieved May 13, 2010.

Total Slave Population in U.S., 1790–1860, by State and Territory

Census Year	1790	1800	1810	1820	1830	1840	1850	1860
All States	694,207	893,308	1,191,338	1,531,490	2,009,079	2,487,392	3,204,215	3,953,820
Alabama	–	494	2,565	41,879	117,549	253,532	342,844	435,080
Arkansas	–	–	136	1,617	4,576	19,935	47,100	111,115
California	–	–	–	–	–	–	0	0
Connecticut	2,648	951	310	97	25	54	0	0
Delaware	8,887	6,153	4,177	4,509	3,292	2,605	2,290	1,798
District of Columbia	–	2,072	3,554	4,520	4,505	3,320	3,687	3,185
Florida	–	–	–	–	15,501	25,717	39,310	61,745
Georgia	29,264	59,699	105,218	149,656	217,531	280,944	381,682	462,198
Illinois	–	107	168	917	747	331	0	0
Indiana	–	28	237	190	3	3	0	0
Iowa	–	–	–	–	–	16	0	0
Kansas	–	–	–	–	–	–	–	2
Kentucky	12,430	40,343	80,561	126,732	165,213	182,258	210,981	225,483
Louisiana	–	–	34,660	69,064	109,588	168,452	244,809	331,726
Maine	–	–	–	–	2	0	0	0
Maryland	103,036	105,635	111,502	107,398	102,994	89,737	90,368	87,189
Massachusetts	0	0	0	0	1	0	0	0
Michigan	–	–	24	0	1	0	0	0
Minnesota	–	–	–	–	–	–	0	0
Mississippi	–	2,995	14,523	32,814	65,659	195,211	309,878	436,631
Missouri	–	–	–	10,222	25,096	58,240	87,422	114,931
Nebraska	–	–	–	–	–	–	–	15
Nevada	–	–	–	–	–	–	–	0
New Hampshire	157	8	0	0	3	1	0	0
New Jersey	11,423	12,422	10,851	7,557	2,254	674	236	18
New York	21,193	20,613	15,017	10,088	75	4	0	0
North Carolina	100,783	133,296	168,824	205,017	245,601	245,817	288,548	331,059
Ohio	–	0	0	0	6	3	0	0
Oregon	–	–	–	–	–	–	0	0
Pennsylvania	3,707	1,706	795	211	403	64	0	0
Rhode Island	958	380	108	48	17	5	0	0
South Carolina	107,094	146,151	196,365	251,783	315,401	327,038	384,984	402,406
Tennessee	3,417	13,584	44,535	80,107	141,603	183,059	239,459	275,719
Texas	–	–	–	–	–	–	58,161	182,566
Utah	–	–	–	–	–	–	26	29
Vermont	0	0	0	0	0	0	0	0
Virginia	287,959	339,499	383,521	411,886	453,698	431,873	452,028	472,494
West Virginia	4,668	7,172	10,836	15,178	17,673	18,488	20,428	18,371
Wisconsin	–	–	–	–	–	11	4	0

For various reasons, the census did not always include all of the slaves, especially in the West. California was admitted as a free state and reported no slaves. However, there were many slaves that were brought to work in the mines during the California Gold Rush. Some Californian communities openly tolerated slavery, such as San Bernardino, which was mostly made up of transplants from the neighboring slave territory of Utah. New Mexico Territory never reported any slaves on the census, yet sued the government for compensation for 600 slaves that were freed when Congress outlawed slavery in the territory. Utah was actively trying to hide its slave population from Congress and did not report slaves in several communities. Additionally, the census did not traditionally include Native Americans, and hence did not include Native American slaves or Native African slaves owned by Native Americans. There were hundreds of Native American slaves in California, Utah and New Mexico that were never recorded in the census.

As of the 1860 census, one may compute the following statistics on slaveholding:

• Enumerating slave schedules by county, 393,975 named persons held 3,950,546 unnamed slaves, for an average of about ten slaves per holder. As some large holders held slaves in multiple counties and are thus multiply counted, this slightly overestimates the number of slaveholders.
• Excluding slaves, the 1860 U.S. population was 27,167,529; therefore, approximately 1.45% of free persons (roughly one in 69) was a named slaveholder (393,975 named slaveholders among 27,167,529 free persons). By counting only named slaveholders, this approach does not acknowledge people who benefited from slavery by being in a slaveowning household, e.g., the wife and children of an owner; in 1850, there was an average of 5.55 people per household, so on average, around 8.05% of free persons lived in a slave-owning household. In the South, 33% of families owned at least one slave. According to historian Joseph Glatthaar, the number of soldiers of the Confederacy's Army of Northern Virginia who either owned slaves or came from slave owning households is "almost one of every two 1861 recruits". In addition he notes that, "Untold numbers of enlistees rented land from, sold crops to, or worked for slaveholders. In the final tabulation, the vast majority of the volunteers of 1861 had a direct connection to slavery."
• It is estimated by the transcriber Tom Blake, that holders of 200 or more slaves, constituting less than 1% of all U.S. slaveholders (fewer than 4,000 persons, one in 7,000 free persons, or 0.015% of the population) held an estimated 20–30% of all slaves (800,000 to 1,200,000 slaves). Nineteen holders of 500 or more slaves have been identified. The largest slaveholder was Joshua John Ward, of Georgetown, South Carolina, who in 1850 held 1,092 slaves, and whose heirs in 1860 held 1,130 or 1,131 slaves – he was dubbed "the king of the rice planters", and one of his plantations is now part of Brookgreen Gardens.
• The percentage of families that owned slaves in 1860 in various groupings of states was as follows:

Group of States	States in Group	Slave-Owning Families
15 states where slavery was legal	Alabama, Arkansas, Delaware, Florida, Georgia, Kentucky, Louisiana, Maryland, Mississippi, Missouri, North Carolina, South Carolina, Tennessee, Texas, Virginia	26%
11 states that seceded	Alabama, Arkansas, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, Tennessee, Texas, Virginia	31%
7 states that seceded before Lincoln's inauguration	Alabama, Florida, Georgia, Louisiana, Mississippi, South Carolina, Texas	37%
4 states that seceded later	Arkansas, North Carolina, Tennessee, Virginia	25%
4 slave states that did not secede	Delaware, Kentucky, Maryland, Missouri	16%

The historian Peter Kolchin, writing in 1993, noted that until the latter decades of the 20th century, historians of slavery had primarily concerned themselves with the culture, practices and economics of the slaveholders, not with the slaves. This was in part due to the circumstance that most slaveholders were literate and left behind written records, whereas slaves were largely illiterate and not in a position to leave written records. Scholars differed as to whether slavery should be considered a benign or a "harshly exploitive" institution.

Much of the history written prior to the 1950s had a distinctive racist slant to it. By the 1970s and 1980s, historians were using archaeological records, black folklore and statistical data to develop a much more detailed and nuanced picture of slave life. Individuals were shown to have been resilient and somewhat autonomous in many of their activities, within the limits of their situation and despite its precariousness. Historians who wrote in this era include John Blassingame (Slave Community), Eugene Genovese (Roll, Jordan, Roll), Leslie Howard Owens (This Species of Property), and Herbert Gutman (The Black Family in Slavery and Freedom).

• Slavery in the Spanish New World colonies
• Slavery in the British and French Caribbean
• Slavery in Cuba
• Slavery in Brazil
• Slavery in Latin America
• Slavery in Canada

• Dickens, Charles (1913). "Slavery". American Notes for General Circulation. First published in 1842. See Louise H. Johnson, "The Source of the Chapter on Slavery in Dickens' American Notes", American Literature, vol. 14, Jan. 1943, pp. 427–430. London: Chapman & Hall.

• Ryan, Tim A. Calls and Responses: The American Novel of Slavery Since Gone with the Wind. Baton Rouge: Louisiana State University Press, 2008.
• Van Deburg, William. Slavery and Race in American Popular Culture. Madison: University of Wisconsin Press, 1984.

• "Traces of the Trade". Traces of the Trade. Retrieved March 19, 2019.

• Slavery in America: A Resource Guide, Library of Congress
• "Born in Slavery: Slave Narratives from the Federal Writers' Project, 1936 to 1938", Library of Congress
• "Voices Remembering Slavery: Freed People Tell Their Stories", audio interviews of former slaves, 1932–1975, Library of Congress
• Digital Library on American Slavery at University of North Carolina at Greensboro
• The Abolitionist Seminar, summaries, lesson plans, documents and illustrations for schools; focus on United States
• American Abolitionism, summaries and documents; focus on United States
• "Slavery and the Making of America", WNET (4-part series)
• Slavery in America at the History Channel
• "Slavery in the United States", Economic History Encyclopedia, March 26, 2008
• North American Slave Narratives, Documenting the American South, Louis Round Wilson Library
• The Trans-Atlantic Slave Trade Database has information on almost 36,000 slaving voyages
• 1850: New Orleans woman and child she held in slavery
• American Capitalism Is Brutal. You Can Trace That to the Plantation. The New York Times Magazine. August 14, 2019.
• The Bibliography of Slavery and World Slaving, University of Virginia: a searchable database of 25,000 scholarly works on slavery and the slave trade in all western European languages.