Shia Islam (/ˈʃiːə/) is the second-largest branch of Islam. It holds that the Islamic prophet Muhammad designated Ali ibn Abi Talib (656–661 CE) as his successor (Arabic: خليفة, romanized: khalīfa) as Imam (امام, 'spiritual and political leader'), most notably at the event of Ghadir Khumm, but that after Muhammad's death, Ali was prevented from succeeding as leader of the Muslims as a result of the choice made by some of Muhammad's other companions (صحابة, ṣaḥāba) at Saqifah. This view primarily contrasts with that of Sunni Islam, whose adherents believe that Muhammad did not appoint a successor before his death and consider Abu Bakr, who was appointed caliph by a group of Muhammad's other companions at Saqifah, to be the first Rashidun ('rightful') caliph after Muhammad (632–634 CE).

Shia Muslims' belief that Ali was the designated successor to Muhammad as Islam's spiritual and political leader later developed into the concept of Imamah, the idea that certain descendants of Muhammad, the Ahl al-Bayt (أَهْل البَيْت, 'People of the House'), are rightful rulers or Imams through the bloodline of Ali and his two sons Hasan and Husayn, whom Shia Muslims believe possess special spiritual and political authority over the Muslim community. Later events such as Husayn's martyrdom in the Battle of Karbala (680 CE) further influenced the development of Shia Islam, contributing to the formation of a distinct religious sect with its own rituals and shared collective memory.

Shia Islam is followed by 10–15% of all Muslims. Although there are many Shia subsects in the Muslim world, Twelver Shi'ism is by far the largest and most influential, comprising about 85% of all Shia Muslims. Others include the Isma'ili, Zaydi, Alevi and Alawi. Shia Muslims form a majority of the population in four countries across the Muslim world: Iran, Iraq, Azerbaijan, and Bahrain. Significant Shia communities are also found in Lebanon, Kuwait, Turkey, Yemen, Saudi Arabia, Afghanistan and the Indian subcontinent. Iran stands as the world's only country where Shia Islam forms the foundation of both its laws and governance system.

The word Shia (or Shīʿa) (/ˈʃiːə/) (Arabic: شيعيّ, romanized: shīʿī, pl. shīʿiyyūn) is derived from شيعة علي, shīʿat ʿAlī, 'followers of Ali'. Shia Islam is also referred to in English as Shiism (or Shīʿism) (/ˈʃiːɪz(ə)m/), and Shia Muslims as Shiites (or Shīʿites) (/ˈʃiːaɪt/).

The term Shia was first used during Muhammad's lifetime. At present, the word refers to the Muslims who believe that the leadership of the Muslim community after Muhammad belongs to ʿAlī ibn Abī Ṭālib, Muhammad's cousin and son-in-law, and his successors. Nawbakhti states that the term Shia refers to a group of Muslims who at the time of Muhammad and after him regarded ʿAlī as the Imam and caliph. Al-Shahrastani expresses that the term Shia refers to those who believe that ʿAlī is designated as the heir, Imam, and caliph by Muhammad and that ʿAlī's authority is maintained through his descendants. For the adherents of Shia Islam, this conviction is implicit in the Quran and the history of Islam. Shia Muslim scholars emphasize that the notion of authority is linked to the family of the Abrahamic prophets as the Quranic verses 3:33 and 3:34 show: "Indeed, Allah chose Adam, Noah, the family of Abraham, and the family of ’Imrân above all people. They are descendants of one another. And Allah is All-Hearing, All-Knowing."

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Shīʿa Islam encompasses various denominations and subgroups, all bound by the belief that the leader of the Muslim community (Ummah) should hail from Ahl al-Bayt, the family of the Islamic prophet Muhammad. It embodies a completely independent system of religious interpretation and political authority in the Muslim world.

Shīʿa Muslims believe that just as a prophet is appointed by God alone, only God has the prerogative to appoint the successor to his prophet. They believe God chose ʿAlī ibn Abī Ṭālib to be Muhammad's successor and the first caliph (Arabic: خليفة, romanized: khalifa) of Islam. Shīʿa Muslims believe that Muhammad designated Ali as his successor by God's command on several instances, but most notably at Eid Al Ghadir. Additionally, ʿAlī was Muhammad's first-cousin and closest living male relative as well as his son-in-law, having married Muhammad's daughter, Fāṭimah.

The Shīʿīte version of the Shahada (Arabic: الشهادة‎), the Islamic profession of faith, differs from that of the Sunnīs. The Sunnī version of the Shahada states La ilaha illallah, Muhammadun rasulullah (Arabic: لَا إِلٰهَ إِلَّا الله مُحَمَّدٌ رَسُولُ الله, lit. 'There is no god except God, Muhammad is the messenger of God'), but in addition to this declaration of faith Shīʿa Muslims add the phrase Ali-un-Waliullah (Arabic: علي ولي الله, lit. 'Ali is the guardian of God'). The basis for the Shīʿīte belief in ʿAlī ibn Abī Ṭālib as the Wali of God is derived from the Qur'anic verse 5:55.

This additional phrase to the declaration of faith embodies the Shīʿīte emphasis on the inheritance of authority through Muhammad's family and lineage. The three clauses of the Shīʿīte version of the Shahada thus address the fundamental Islamic beliefs of Tawḥīd (Arabic: تَوْحِيد, lit. 'oneness of God'), Nubuwwah (Arabic: نبوة, lit. 'prophethood'), and Imamah (Arabic: إمامة, lit. 'Imamate or leadership').

Ismah (Arabic: عِصْمَة, romanized: 'Iṣmah or 'Isma, lit. 'protection') is the concept of infallibility or "divinely bestowed freedom from error and sin" in Islam. Muslims believe that Muhammad, along with the other prophets and messengers, possessed ismah. Twelver and Ismāʿīlī Shīʿa Muslims also attribute the quality to Imams as well as to Fāṭimah, daughter of Muhammad, in contrast to the Zaydī Shīʿas, who do not attribute ismah to the Imams. Though initially beginning as a political movement, infallibility and sinlessness of the Imams later evolved as a distinct belief of (non-Zaydī) Shīʿīsm.

According to Shīʿa Muslim theologians, infallibility is considered a rational, necessary precondition for spiritual and religious guidance. They argue that since God has commanded absolute obedience from these figures, they must only order that which is right. The state of infallibility is based on the Shīʿīte interpretation of the verse of purification. Thus, they are the most pure ones, the only immaculate ones preserved from, and immune to, all uncleanness. It does not mean that supernatural powers prevent them from committing a sin, but due to the fact that they have absolute belief in God, they refrain from doing anything that is a sin.

They also have a complete knowledge of God's will. They are in possession of all knowledge brought by the angels (Arabic: ملائِكة, romanized: malāʾikah) to the prophets (Arabic: أنبياء, romanized: anbiyāʼ) and the messengers (Arabic: رُسل, romanized: rusul). Their knowledge encompasses the totality of all times. Thus, they are believed to act without fault in religious matters. Shi'a Muslims regard ʿAlī ibn Abī Ṭālib as the successor of Muhammad not only ruling over the entire Muslim community in justice, but also in interpreting the Islamic faith, practices, and its esoteric meaning. ʿAlī is regarded as a "perfect man" (Arabic: الإنسان الكامل, romanized: al-insan al-kamil) similar to Muhammad, according to the Shīʿīte perspective.

The Occultation is an eschatological belief held in various denominations of Shīʿa Islam concerning a messianic figure, the hidden and last Imam known as "the Mahdi", that one day shall return on Earth and fill the world with justice. According to the doctrine of Twelver Shīʿīsm, the main goal of Imam Mahdi will be to establish an Islamic state and to apply Islamic laws that were revealed to Muhammad. The Quran does not contain verses on the Imamate, which is the basic doctrine of Shīʿa Islam. Some Shīʿa subsects, such as the Zaydī Shīʿas and Nizārī Ismāʿīlīs, do not believe in the idea of the Occultation. The groups which do believe in it differ as to which lineage of the Imamate is valid, and therefore which individual has gone into Occultation. They believe there are many signs that will indicate the time of his return.

Twelver Shīʿa Muslims believe that the prophesied Mahdi and 12th Shīʿīte Imam, Hujjat Allah al-Mahdi, is already on Earth in Occultation, and will return at the end of time. Ṭayyibi Ismāʿīlīs and Fatimid/Bohra/Dawoodi Bohra believe the same but for their 21st Ṭayyib, At-Tayyib Abi l-Qasim, and also believe that a Da'i al-Mutlaq ("Unrestricted Missionary") maintains contact with him. Sunnī Muslims believe that the future Mahdi has not yet arrived on Earth.

Shīʿa Muslims believe that the status of ʿAlī is supported by numerous ḥadīth reports, including the Hadith of the pond of Khumm, Hadith of the two weighty things, Hadith of the pen and paper, Hadith of the invitation of the close families, and Hadith of the Twelve Successors. In particular, the Hadith of the Cloak is often quoted to illustrate Muhammad's feeling towards ʿAlī and his family by both Sunnī and Shia scholars. Shia Muslims prefer to study and read the hadith attributed to the Ahl al-Bayt and close associates, and most have their own separate hadith canon.

Shīʿa Muslims believe that the armaments and sacred items of all of the Abrahamic prophets, including Muhammad, were handed down in succession to the Imams of the Ahl al-Bayt. Jaʿfar al-Ṣādiq, the 6th Shīʿīte Imam, in Kitab al-Kafi mentions that "with me are the arms of the Messenger of Allah. It is not disputable."

Further, he claims that with him is the sword of the Messenger of God, his coat of arms, his Lamam (pennon) and his helmet. In addition, he mentions that with him is the flag of the Messenger of God, the victorious. With him is the Staff of Moses, the ring of Solomon, son of David, and the tray on which Moses used to offer his offerings. With him is the name that whenever the Messenger of God would place it between the Muslims and pagans no arrow from the pagans would reach the Muslims. With him is the similar object that angels brought.

Al-Ṣādiq also narrated that the passing down of armaments is synonymous to receiving the Imamat (leadership), similar to how the Ark of Covenant in the house of the Israelites signaled prophethood. Imam Ali al-Ridha narrates that wherever the armaments among us would go, knowledge would also follow and the armaments would never depart from those with knowledge (Imamat).

According to Muhammad Rida al-Muzaffar, God gives humans the faculty of reason and argument. Also, God orders humans to spend time thinking carefully on creation while he refers to all creations as his signs of power and glory. These signs encompass all of the universe. Furthermore, there is a similarity between humans as the little world and the universe as the large world. God does not accept the faith of those who follow him without thinking and only with imitation, but also God blames them for such actions. In other words, humans have to think about the universe with reason and intellect, a faculty bestowed on us by God. Since there is more insistence on the faculty of intellect among Shia Muslims, even evaluating the claims of someone who claims prophecy is on the basis of intellect.

Shia religious practices, such as prayers, differ only slightly from the Sunnīs. While all Muslims pray five times daily, Shia Muslims have the option of combining Dhuhr with Asr and Maghrib with Isha', as there are three distinct times mentioned in the Quran. The Sunnīs tend to combine only under certain circumstances.

Shia Muslims celebrate the following annual holidays:

• Eid ul-Fitr, which marks the end of fasting during the month of Ramadan
• Eid al-Adha, which marks the end of the Hajj or pilgrimage to Mecca
• Eid al-Ghadeer, which is the anniversary of the Ghadir Khum, the occasion when Muhammad announced Ali's Imamate before a multitude of Muslims. Eid al-Ghadeer is held on the 18th of Dhu al-Hijjah.
• The Mourning of Muharram and the Day of Ashura for Shia Muslims commemorate the martyrdom of Ḥusayn ibn ʿAlī, brother of Ḥasan and grandson of Muhammad, who was killed by Yazid ibn Muawiyah in Karbala (central Iraq). Ashura is a day of deep mourning which occurs on the 10th of Muharram.
• Arba'een commemorates the suffering of the women and children of Ḥusayn ibn ʿAlī's household. After Ḥusayn was killed, they were marched over the desert, from Karbala (central Iraq) to Shaam (Damascus, Syria). Many children (some of whom were direct descendants of Muhammad) died of thirst and exposure along the route. Arbaein occurs on the 20th of Safar, 40 days after Ashura.
• Mawlid, Muhammad's birth date. Unlike Sunnī Muslims, who celebrate the 12th of Rabi' al-awwal as Muhammad's day of birth or death (because they assert that his birth and death both occur in this week), Shia Muslims celebrate Muhammad's birthday on the 17th of the month, which coincides with the birth date of Jaʿfar al-Ṣādiq, the 6th Shīʿīte Imam.
• Fāṭimah's birthday on 20th of Jumada al-Thani. This day is also considered as the "'women and mothers' day"
• ʿAlī's birthday on 13th of Rajab.
• Mid-Sha'ban is the birth date of the 12th and final Twelver imam, Muhammad al-Mahdi. It is celebrated by Shia Muslims on the 15th of Sha'aban.
• Laylat al-Qadr, anniversary of the night of the revelation of the Quran.
• Eid al-Mubahila celebrates a meeting between the Ahl al-Bayt (household of Muhammad) and a Christian deputation from Najran. Al-Mubahila is held on the 24th of Dhu al-Hijjah.

After Mecca and Medina, the two holiest cities of Islam, the cities of Najaf, Karbala, Mashhad and Qom are the most revered by Shīʿa Muslims. The Sanctuary of Imām ʿAlī in Najaf, the Shrine of Imam Ḥusayn in Karbala, The Sanctuary of Imam Reza in Mashhad and the Shrine of Fāṭimah al-Maʿṣūmah in Qom are very essential for Shīʿa Muslims. Other venerated pilgrimage sites include the Kadhimiya Mosque in Kadhimiya, Al-Askari Mosque in Samarra, the Sahla Mosque, the Great Mosque of Kufa, the Jamkaran Mosque in Qom, and the Tomb of Daniel in Susa.

Most of the Shīʿa sacred places and heritage sites in Saudi Arabia have been destroyed by the Al Saud-Wahhabi armies of the Ikhwan, the most notable being the tombs of the Imams located in the Al-Baqi' cemetery in 1925. In 2006, a bomb destroyed the shrine of Al-Askari Mosque. (See: Anti-Shi'ism).

Shia orthodoxy, particularly in Twelver Shi'ism, has considered non-Muslims as agents of impurity (Najāsat). This categorization sometimes extends to kitābῑ, individuals belonging to the People of the Book, with Jews explicitly labeled as impure by certain Shia religious scholars. Armenians in Iran, who have historically played a crucial role in the Iranian economy, received relatively more lenient treatment.

Shi'ite theologians and mujtahids (jurists), such as Muḥammad Bāqir al-Majlisῑ, held that Jews' impurity extended to the point where they were advised to stay at home on rainy or snowy days to prevent contaminating their Shia neighbors. Ayatollah Khomeini, Supreme Leader of Iran from 1979 to 1989, asserted that every part of an unbeliever's body, including hair, nails, and bodily secretions, is impure. However, the current leader of Iran, ʿAlī Khameneʾī, stated in a fatwa that Jews and other Peoples of the Book are not inherently impure, and touching the moisture on their hands does not convey impurity.

The original Shia identity referred to the followers of Imam ʿAlī, and Shia theology was formulated after the hijra (8th century CE). The first Shia governments and societies were established by the end of the 9th century CE. The 10th century CE has been referred to by the scholar of Islamic studies Louis Massignon as "the Shiite Ismaili century in the history of Islam".

The Shia, originally known as the "partisans" of ʿAlī ibn Abī Ṭālib, Muhammad's cousin and Fatima's husband, first emerged as a distinct movement during the First Fitna from 656 to 661 CE. Shia doctrine holds that ʿAlī was meant to lead the community after Muhammad's death in 632. Historians dispute over the origins of Shia Islam, with many Western scholars positing that Shīʿīsm began as a political faction rather than a truly religious movement. Other scholars disagree, considering this concept of religious-political separation to be an anachronistic application of a Western concept.

Shia Muslims believe that Muhammad designated ʿAlī ibn Abī Ṭālib as his heir during a speech at Ghadir Khumm. The point of contention between different Muslim sects arises when Muhammad, whilst giving his speech, gave the proclamation "Anyone who has me as his mawla, has ʿAlī as his mawla". Some versions add the additional sentence "O God, befriend the friend of ʿAlī and be the enemy of his enemy". Sunnis maintain that Muhammad emphasized the deserving friendship and respect for ʿAlī. In contrast, Shia Muslims assert that the statement unequivocally designates ʿAlī as Muhammad's appointed successor. Shia sources also record further details of the event, such as stating that those present congratulated ʿAlī and acclaimed him as Amir al-Mu'minin ("commander of the believers").

When Muhammad died in 632 CE, ʿAlī ibn Abī Ṭālib and Muhammad's closest relatives made the funeral arrangements. While they were preparing his body, Abū Bakr, ʿUmar ibn al-Khaṭṭāb, and Abu Ubaidah ibn al Jarrah met with the leaders of Medina and elected Abū Bakr as the first rāshidūn caliph. Abū Bakr served from 632 to 634, and was followed by Umar (634–644) and ʿUthmān (644–656).

With the murder of ʿUthmān in 657 CE, the Muslims of Medina invited ʿAlī to become the fourth caliph as the last source, and he established his capital in Kufa. ʿAlī's rule over the early Islamic empire, between 656 CE to 661 CE, was often contested. Tensions eventually led to the First Fitna, the first major civil war between Muslims within the empire, which began as a series of revolts fought against ʿAlī. While the rebels had previously affirmed the legitimacy of ʿAlī's khilafāʾ (caliphate), they later turned against ʿAlī and fought him. Tensions escalated into the Battle of the Camel in 656, where Ali's forces emerged victorious against Aisha, Talhah, and al-Zubayr. However, the Battle of Siffin in 657 turned the tide against ʿAlī, who lost due to arbitration issues with Muawiyah, the governor of Damascus. ʿAlī withdrew to Kufa, overcoming the Kharijis, a faction that had transformed from supporters to bitter rivals, at Nahrawan in 658. In 661, ʿAlī was assassinated by a Khariji assassin in Kufa while in the act of prostration during prayer (sujud). Subsequently, Muawiyah asserted his claim to the caliphate.

Upon the death of ʿAlī, his elder son Ḥasan became leader of the Muslims of Kufa. After a series of skirmishes between the Kufa Muslims and the army of Muawiyah, Ḥasan ibn Ali agreed to cede the caliphate to Muawiyah and maintain peace among Muslims upon certain conditions: The enforced public cursing of ʿAlī, e.g. during prayers, should be abandoned; Muawiyah should not use tax money for his own private needs; There should be peace, and followers of Ḥasan should be given security and their rights; Muawiyah will never adopt the title of Amir al-Mu'minin ("commander of the believers"); Muawiyah will not nominate any successor. Ḥasan then retired to Medina, where in 670 CE he was poisoned by his wife Ja'da bint al-Ash'ath, after being secretly contacted by Muawiyah who wished to pass the caliphate to his own son Yazid and saw Ḥasan as an obstacle.

Ḥusayn ibn ʿAlī, ʿAlī's younger son and brother to Ḥasan, initially resisted calls to lead the Muslims against Muawiyah and reclaim the caliphate. In 680 CE, Muawiyah died and passed the caliphate to his son Yazid, and breaking the treaty with Ḥasan ibn ʿAlī. Yazid asked Husayn to swear allegiance (bay'ah) to him. ʿAlī's faction, having expected the caliphate to return to ʿAlī's line upon Muawiyah's death, saw this as a betrayal of the peace treaty and so Ḥusayn rejected this request for allegiance. There was a groundswell of support in Kufa for Ḥusayn to return there and take his position as caliph and Imam, so Ḥusayn collected his family and followers in Medina and set off for Kufa.

En route to Kufa, Husayn was blocked by an army of Yazid's men, which included people from Kufa, near Karbala; rather than surrendering, Husayn and his followers chose to fight. In the Battle of Karbala, Ḥusayn and approximately 72 of his family members and followers were killed, and Husayn's head was delivered to Yazid in Damascus. The Shi'a community regard Ḥusayn ibn ʿAlī as a martyr (shahid), and count him as an Imam from the Ahl al-Bayt. The Battle of Karbala and martyrdom of Ḥusayn ibn ʿAlī is often cited as the definitive separation between the Shia and Sunnī sects of Islam. Ḥusayn is the last Imam following ʿAlī mutually recognized by all branches of Shia Islam. The martyrdom of Husayn and his followers is commemorated on the Day of Ashura, occurring on the tenth day of Muharram, the first month of the Islamic calendar.

Later, most denominations of Shia Islam, including Twelvers and Ismāʿīlīs, became Imamis. Imami Shīʿītes believe that Imams are the spiritual and political successors to Muhammad. Imams are human individuals who not only rule over the Muslim community with justice, but also are able to keep and interpret the divine law and its esoteric meaning. The words and deeds of Muhammad and the Imams are a guide and model for the community to follow; as a result, they must be free from error and sin, and must be chosen by divine decree (nass) through Muhammad. According to this view peculiar to Shia Islam, there is always an Imam of the Age, who is the divinely appointed authority on all matters of faith and law in the Muslim community. ʿAlī was the first Imam of this line, the rightful successor to Muhammad, followed by male descendants of Muhammad through his daughter Fatimah.

This difference between following either the Ahl al-Bayt (Muhammad's family and descendants) or pledging allegiance to Abū Bakr has shaped the Shia–Sunnī divide on the interpretation of some Quranic verses, hadith literature (accounts of the sayings and living habits attributed to the Islamic prophet Muhammad during his lifetime), and other areas of Islamic belief throughout the history of Islam. For instance, the hadith collections venerated by Shia Muslims are centered on narrations by members of the Ahl al-Bayt and their supporters, while some hadith transmitted by narrators not belonging to or supporting the Ahl al-Bayt are not included. Those of Abu Hurairah, for example, Ibn Asakir in his Taʿrikh Kabir, and Muttaqi in his Kanzuʿl-Umma report that ʿUmar ibn al-Khaṭṭāb lashed him, rebuked him, and forbade him to narrate ḥadīth from Muhammad. ʿUmar is reported to have said: "Because you narrate hadith in large numbers from the Holy Prophet, you are fit only for attributing lies to him. (That is, one expects a wicked man like you to utter only lies about the Holy Prophet.) So you must stop narrating hadith from the Prophet; otherwise, I will send you to the land of Dus." (An Arab clan in Yemen, to which Abu Hurairah belonged). According to Sunnī Muslims, ʿAlī was the fourth successor to Abū Bakr, while Shia Muslims maintain that ʿAlī was the first divinely sanctioned "Imam", or successor of Muhammad. The seminal event in Shia history is the martyrdom at the Battle of Karbala of ʿAlī's son, Ḥusayn ibn ʿAlī, and 71 of his followers in 680 CE, who led a non-allegiance movement against the defiant caliph.

It is believed in Twelver and Ismāʿīlī branches of Shia Islam that divine wisdom (ʿaql) was the source of the souls of the prophets and Imams, which bestowed upon them esoteric knowledge (ḥikmah), and that their sufferings were a means of divine grace to their devotees. Although the Imam was not the recipient of a divine revelation (waḥy), he had a close relationship with God, through which God guides him, and the Imam, in turn, guides the people. Imamate, or belief in the divine guide, is a fundamental belief in the Twelver and Ismāʿīlī branches of Shia Islam, and is based on the concept that God would not leave humanity without access to divine guidance.

In Shia Islam, Imam Mahdi is regarded as the prophesied eschatological redeemer of Islam who will rule for seven, nine, or nineteen years (according to differing interpretations) before the Day of Judgment and will rid the world of evil. According to Islamic tradition, the Mahdi's tenure will coincide with the Second Coming of Jesus (ʿĪsā), who is to assist the Mahdi against the Masih ad-Dajjal (literally, the "false Messiah" or Antichrist). Jesus, who is considered the Masih ("Messiah") in Islam, will descend at the point of a white arcade east of Damascus, dressed in yellow robes with his head anointed. He will then join the Mahdi in his war against the Dajjal, where it is believed the Mahdi will slay the Dajjal and unite humankind.

In the century following the Battle of Karbala (680 CE), as various Shia-affiliated groups diffused in the emerging Islamic world, several nations arose based on a Shia leadership or population.

• Idrisids (788–985 CE): a Zaydi dynasty in what is now Morocco.
• Qarmatians (899–1077 CE): an Ismaili Iranian dynasty. Their headquarters were in Eastern Arabia and Bahrain. It was founded by Abu Sa'id al-Jannabi.
• Buyids (934–1055 CE): a Twelver Iranian dynasty. at its peak consisted of large portions of Iran and Iraq.
• Uqaylids (990–1096 CE): a Shia Arab dynasty with several lines that ruled in various parts of al-Jazira, northern Syria and Iraq.
• Ilkhanate (1256–1335): a Persianate Mongol khanate established in Iran in the 13th century, considered a part of the Mongol Empire. The Ilkhanate was based, originally, on Genghis Khan's campaigns in the Khwarezmid Empire in 1219–1224, and founded by Genghis's grandson, Hulagu, in territories in Western and Central Asia which today comprise most of Iran, Iraq, Afghanistan, Turkmenistan, Armenia, Azerbaijan, Georgia, Turkey, and Pakistan. The Ilkhanate initially embraced many religions, but was particularly sympathetic to Buddhism and Christianity. Later Ilkhanate rulers, beginning with Ghazan in 1295, chose Islam as the state religion; his brother Öljaitü promoted Shia Islam.
• Bahmanids (1347–1527): a Shia Muslim state of the Deccan Plateau in Southern India, and one of the great medieval Indian kingdoms. Bahmanid Sultanate was the first independent Islamic kingdom in Southern India.

• Fatimids (909–1171 CE): Controlled much of North Africa, the Levant, parts of Arabia, and the holy cities of Mecca and Medina. The group takes its name from Fāṭimah, Muhammad's daughter, from whom they claim descent.
  • In 909 CE, the Shia military leader Abu Abdallah al-Shiʻi overthrew the Sunni rulers in North Africa, an event which led to the foundation of the Fatimid Caliphate.
  • Al-Qaid Jawhar ibn Abdallah (Arabic: جوهر; fl. 966–d. 992) was a Shia Fatimid general. Under the command of Caliph al-Muʻizz, he led the conquest of North Africa and then of Egypt, founded the city of Cairo and the al-Azhar Mosque. A Greek slave by origin, he was freed by al-Muʻizz.

A major turning point in the history of Shia Islam was the dominion of the Safavid dynasty (1501–1736) in Persia. This caused a number of changes in the Muslim world:

• The ending of the relative mutual tolerance between Sunnīs and Shias that existed from the time of the Mongol conquests onwards and the resurgence of antagonism between the two groups.
• Initial dependence of Shīʿīte clerics on the state followed by the emergence of an independent body of ulama capable of taking a political stand different from official policies.
• The growth in importance of Persian centers of Islamic education and religious learning, which resulted in the change of Twelver Shīʿīsm from being a predominantly Arab phenomenon to become predominantly Persian.
• The growth of the Akhbari school of thought, which taught that only the Quran, ḥadīth literature, and sunnah (accounts of the sayings and living habits attributed to the Islamic prophet Muhammad during his lifetime) are to be bases for verdicts, rejecting the use of reasoning.

With the fall of the Safavids, the state in Iran—including the state system of courts with government-appointed judges (qāḍī)—became much weaker. This gave the sharīʿa courts of mujtahid an opportunity to fill the legal vacuum and enabled the ulama to assert their judicial authority. The Usuli school of thought also increased in strength at this time.

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  The declaration of Twelver Shīʿīsm as the state religion of Safavids
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  Battle of Chaldiran in 1514 was a major sectarian crisis between Muslims in the Middle East.
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  Monument commemorating the Battle of Chaldiran, where more than 7,000 Muslims of the Shia and Sunnī sects killed each other

Shia Islam is the second largest branch of Islam. It is estimated that either 10–20% or 10–13% of the global Muslim population are Shias. They may number up to 200 million as of 2009. As of 1985, Shia Muslims are estimated to be 21% of the Muslim population in South Asia, although the total number is difficult to estimate.

Shia Muslims form a distinct majority of the population in three countries of the Muslim world: Iran, Iraq, and Azerbaijan. Shia Muslims constitute 36.3% of the entire population (and 38.6% of the Muslim population) of the Middle East.

Estimates have placed the proportion of Shia Muslims in Lebanon between 27% and 45% of the population, 30–35% of the citizen population in Kuwait (no figures exist for the non-citizen population), over 20% in Turkey, 5–20% of the population in Pakistan, and 10–19% of Afghanistan's population, and 45% in Bahrain.

Saudi Arabia hosts a number of distinct Shia communities, including the Twelver Baharna in the Eastern Province and Nakhawila of Medina, and the Ismāʿīlī Sulaymani and Zaydī Shias of Najran. Estimations put the number of Shīʿīte citizens at roughly 15% of the local population. Approximately 40% of the population of Yemen are Shia Muslims.

Significant Shia communities also exist in the coastal regions of West Sumatra and Aceh in Indonesia (see Tabuik). The Shia presence is negligible elsewhere in Southeast Asia, where Muslims are predominantly Shāfiʿī Sunnīs.

A significant Shia minority is present in Nigeria, made up of modern-era converts to a Shīʿīte movement centered around Kano and Sokoto states. Several African countries like Kenya, South Africa, Somalia, etc. hold small minority populations of various Shia subsects, primarily descendants of immigrants from South Asia during the colonial period, such as the Khoja.

Figures indicated in the first three columns below are based on the October 2009 demographic study by the Pew Research Center report, Mapping the Global Muslim Population.

Nations with over 100,000 Shia

Country	Article	Shia population in 2009 (Pew)	Percent of population that is Shia in 2009 (Pew)	Percent of global Shia population in 2009 (Pew)	Population estimate ranges and notes
Iran	Islam in Iran	66,000,000–69,500,000	90–95	37–40
Pakistan	Shia Islam in the Indian subcontinent	25,272,000	15	15	A 2010 estimate was that Shia made up about 10–15% of Pakistan's population.
Iraq	Shi'a Islam in Iraq	19,000,000–24,000,000	55–65	10–11
India	Shia Islam in the Indian subcontinent	12,300,000–18,500,000	1.3–2	9–14
Yemen	Shia Islam in Yemen	7,000,000–8,000,000	35–40	~5	Majority following Zaydi Shia sect.
Turkey	Shi'a Islam in Turkey	6,000,000–9,000,000	~10–15	~3–4	Majority following Alevi Shia sect.
Azerbaijan	Islam in Azerbaijan	4,575,000–5,590,000	45–55	2–3	Azerbaijan is majority Shia. A 2012 work noted that in Azerbaijan, among believers of all faiths, 10% identified as Sunni, 30% identified as Shia, and the remainder of followers of Islam simply identified as Muslim.
Afghanistan	Shi'a Islam in Afghanistan	3,000,000	15	~2	A reliable census has not been taken in Afghanistan in decades, but about 20% of Afghan population is Shia, mostly among ethnic Tajik and Hazara minorities.
Syria	Islam in Syria	2,400,000	13	~2	Majority following Alawites Shia sect.
Lebanon	Shi'a Islam in Lebanon	2,100,000	31.2	<1	In 2020, the CIA World Factbook stated that Shia Muslims constitute 31.2% of Lebanon's population.
Saudi Arabia	Shi'a Islam in Saudi Arabia	2,000,000	~6
Nigeria	Shi'a Islam in Nigeria	<2,000,000	<1	<1	Estimates range from as low as 2% of Nigeria's Muslim population to as high as 17% of Nigeria's Muslim population. Some, but not all, Nigerian Shia are affiliated with the banned Islamic Movement in Nigeria, an Iranian-inspired Shia organization led by Ibrahim Zakzaky.
Tanzania	Islam in Tanzania	~1,500,000	~2.5	<1
Kuwait	Shi'a Islam in Kuwait	500,000–700,000	20–25	<1	Among Kuwait's estimated 1.4 million citizens, about 30% are Shia (including Ismaili and Ahmadi, whom the Kuwaiti government count as Shia). Among Kuwait's large expatriate community of 3.3 million noncitizens, about 64% are Muslim, and among expatriate Muslims, about 5% are Shia.
Bahrain	Islam in Bahrain	400,000–500,000	65–70	<1
Tajikistan	Shi'a Islam in Tajikistan	~400,000	~4	<1	Shi'a Muslims in Tajikistan are predominantly Nizari Ismaili
Germany	Islam in Germany	~400,000	~0.5	<1
United Arab Emirates	Islam in the United Arab Emirates	~300,000	~3	<1
United States	Islam in the United States Shia Islam in the Americas	~225,000	~0.07	<1	Shi'a form a majority amongst Arab Muslims in many American cities, e.g. Lebanese Shi'a forming the majority in Detroit.
United Kingdom	Islam in the United Kingdom	~125,000	~0.2	<1
Qatar	Islam in Qatar	~100,000	~3.5	<1
Oman	Islam in Oman	~100,000	~2	<1	As of 2015, about 5% of Omanis are Shia (compared to about 50% Ibadi and 45% Sunni).

The Shia community throughout its history split over the issue of the Imamate. The largest branch are the Twelvers, followed by the Zaydīs and the Ismāʿīlīs. Each subsect of Shīʿīsm follows its own line of Imamate. All mainstream Twelver and Ismāʿīlī Shia Muslims follow the same school of thought, the Jaʽfari jurisprudence, named after Jaʿfar al-Ṣādiq, the 6th Shīʿīte Imam. Shīʿīte clergymen and jurists usually carry the title of mujtahid (i.e., someone authorized to issue legal opinions in Shia Islam).

Twelver Shīʿīsm or Ithnāʿashariyyah is the largest branch of Shia Islam, and the terms Shia Muslim and Shia often refer to the Twelvers by default. The designation Twelver is derived from the doctrine of believing in twelve divinely ordained leaders, known as "the Twelve Imams". Twelver Shia are otherwise known as Imami or Jaʿfari; the latter term derives from Jaʿfar al-Ṣādiq, the 6th Shīʿīte Imam, who elaborated the Twelver jurisprudence. Twelver Shia constitute the majority of the population in Iran (90%), Azerbaijan (85%), Bahrain (70%), Iraq (65%), and Lebanon (65% of Muslims).

Twelver doctrine is based on five principles. These five principles known as Usul ad-Din are as follow:

1. Monotheism: God is one and unique;
2. Justice: the concept of moral rightness based on ethics, fairness, and equity, along with the punishment of the breach of these ethics;
3. Prophethood: the institution by which God sends emissaries, or prophets, to guide humankind;
4. Leadership: a divine institution which succeeded the institution of Prophethood. Its appointees (Imams) are divinely appointed;
5. Resurrection and Last Judgment: God's final assessment of humanity.

Besides the Quran, which is the sacred text common to all Muslims, Twelver Shias derive scriptural and authoritative guidance from collections of sayings and traditions (hadith) attributed to Muhammad and the Twelve Imams. Below is a list of some of the most prominent of these books:

• Nahj al-Balagha by Ash-Sharif Ar-Radhi – the most famous collection of sermons, letters & narration attributed to Ali, the first Imam regarded by Shias
• Kitab al-Kafi by Muhammad ibn Ya'qub al-Kulayni
• Wasa'il al-Shiʻah by al-Hurr al-Amili

According to the theology of Twelvers, the successor of Muhammad is an infallible human individual who not only rules over the Muslim community with justice but also is able to keep and interpret the divine law (sharīʿa) and its esoteric meaning. The words and deeds of Muhammad and the Twelve Imams are a guide and model for the Muslim community to follow; as a result, they must be free from error and sin, and Imams must be chosen by divine decree (nass) through Muhammad. The twelfth and final Imam is Hujjat Allah al-Mahdi, who is believed by Twelvers to be currently alive and hidden in Occultation.

The Twelver jurisprudence is called Jaʽfari jurisprudence. In this school of Islamic jurisprudence, the sunnah is considered to be comprehensive of the oral traditions of Muhammad and their implementation and interpretation by the Twelve Imams. There are three schools of Jaʿfari jurisprudence: Usuli, Akhbari, and Shaykhi; the Usuli school is by far the largest of the three. Twelver groups that do not follow the Jaʿfari jurisprudence include Alevis, Bektashi, and Qizilbash.

The five pillars of Islam to the Jaʿfari jurisprudence are known as Usul ad-Din:

1. Tawḥīd: unity and oneness of God;
2. Nubuwwah: prophethood of Muhammad;
3. Muʿad: resurrection and final judgment;
4. ʿAdl: justice of God;
5. Imamah: the rightful place of the Shīʿīte Imams.

In Jaʿfari jurisprudence, there are eight secondary pillars, known as Furu ad-Din, which are as follows:

1. Salat (prayer);
2. Sawm (fasting);
3. Hajj (pilgrimage) to Mecca;
4. Zakāt (alms giving to the poor);
5. Jihād (struggle) for the righteous cause;
6. Directing others towards good;
7. Directing others away from evil;
8. Khums (20% tax on savings yearly, after deduction of commercial expenses).

According to Twelvers, defining and interpretation of Islamic jurisprudence (fiqh) is the responsibility of Muhammad and the Twelve Imams. Since the 12th Imam is currently in Occultation, it is the duty of Shīʿīte clerics to refer to the Islamic literature, such as the Quran and hadith, and identify legal decisions within the confines of Islamic law to provide means to deal with current issues from an Islamic perspective. In other words, clergymen in Twelver Shīʿīsm are believed to be the guardians of fiqh, which is believed to have been defined by Muhammad and his twelve successors. This process is known as ijtihad and the clerics are known as marjaʿ, meaning "reference"; the labels Allamah and Ayatollah are in use for Twelver clerics.

Islamist Shīʿīsm (Persian: تشیع اخوانی) is a new denomination within Twelver Shīʿīsm greatly inspired by the political ideology of the Muslim Brotherhood and mysticism of Ibn Arabi. It sees Islam as a political system and differs from the other mainstream Usuli and Akhbari groups in favoring the idea of the establishment of an Islamic state in Occultation under the rule of the 12th Imam. Hadi Khosroshahi was the first person to identify himself as ikhwani (Islamist) Shia Muslim. Because of the concept of the hidden Imam, Muhammad al-Mahdi, Shia Islam is inherently secular in the age of Occultation, therefore Islamist Shia Muslims had to borrow ideas from Sunnī Islamists and adjust them in accordance with the doctrine of Shīʿīsm. Its foundations were laid during the Persian Constitutional Revolution at the start of 20th century in Qajar Empire (1905–1911), when Fazlullah Nouri supported the Persian king Ahmad Shah Qajar against the will of Muhammad Kazim Khurasani, the Usuli marjaʿ of the time.

Ismāʿīlīs, otherwise known as Sevener, derive their name from their acceptance of Ismāʿīl ibn Jaʿfar as the divinely appointed spiritual successor (Imam) to Jaʿfar al-Ṣādiq, the 6th Shīʿīte Imam, wherein they differ from the Twelvers, who recognize Mūsā al-Kāẓim, younger brother of Ismāʿīl, as the true Imam.

After the death or Occultation of Muhammad ibn Imam Ismāʿīl in the 8th century CE, the teachings of Ismāʿīlīsm further transformed into the belief system as it is known today, with an explicit concentration on the deeper, esoteric meaning (bāṭin) of the Islamic faith. With the eventual development of Twelver Shīʿīsm into the more literalistic (zahīr) oriented Akhbari and later Usuli schools of thought, Shīʿīsm further developed in two separate directions: the metaphorical Ismāʿīlī group focusing on the mystical path and nature of God and the divine manifestation in the personage of the "Imam of the Time" as the "Face of God", with the more literalistic Twelver group focusing on divine law (sharī'ah) and the deeds and sayings (sunnah) attributed to Muhammad and his successors (the Ahl al-Bayt), who as A'immah were guides and a light (nūr) to God.

Though there are several subsects amongst the Ismāʿīlīs, the term in today's vernacular generally refers to the Shia Imami Ismāʿīlī Nizārī community, often referred to as the Ismāʿīlīs by default, who are followers of the Aga Khan and the largest group within Ismāʿīlīsm. Another Shia Imami Ismāʿīlī community are the Dawudi Bohras, led by a Da'i al-Mutlaq ("Unrestricted Missionary") as representative of a hidden Imam. While there are many other branches with extremely differing exterior practices, much of the spiritual theology has remained the same since the days of the faith's early Imams. In recent centuries, Ismāʿīlīs have largely been an Indo-Iranian community, but they can also be found in India, Pakistan, Syria, Palestine, Saudi Arabia, Yemen, Jordan, Uzbekistan, Tajikistan, Afghanistan, East and South Africa, and in recent years several Ismāʿīlīs have emigrated to China, Western Europe (primarily in the United Kingdom), Australia, New Zealand, and North America.

In the Nizārī Ismāʿīlī interpretation of Shia Islam, the Imam is the guide and the intercessor between humans and God, and the individual through whom God is recognized. He is also responsible for the esoteric interpretation of the Quran (taʾwīl). He is the possessor of divine knowledge and therefore the "Prime Teacher". According to the "Epistle of the Right Path", a Persian Ismāʿīlī prose text from the post-Mongol period of Ismāʿīlī history, by an anonymous author, there has been a chain of Imams since the beginning of time, and there will continue to be an Imam present on the Earth until the end of time. The worlds would not exist in perfection without this uninterrupted chain of Imams. The proof (hujja) and gate (bāb) of the Imam are always aware of his presence and are witness to this uninterrupted chain.

After the death of Ismāʿīl ibn Jaʿfar, many Ismāʿīlīs believed that one day the eschatological figure of Imam Mahdi, whom they believed to be Muhammad ibn Imam Ismāʿīl, would return and establish an age of justice. One group included the violent Qarmatians, who had a stronghold in Bahrain. In contrast, some Ismāʿīlīs believed the Imamate did continue, and that the Imams were in Occultation and still communicated and taught their followers through a network of Da'i ("Missionaries").

In 909 CE, Abdullah al-Mahdi Billah, a claimant to the Ismāʿīlī Imamate, established the Fatimid Caliphate. During this period, three lineages of Imams were formed. The first branch, known today as the Druze, began with Al-Ḥākim bi-Amr Allāh. Born in 985 CE, he ascended as ruler at the age of eleven. When in 1021 CE his mule returned without him, soaked in blood, a religious group that was forming in his lifetime broke off from mainstream Ismāʿīlīsm and did not acknowledge his successor. Later to be known as the Druze, they believe Al-Ḥākim to be God incarnate and the prophesied Mahdi on Earth, who would one day return and bring justice to the world. The Druze faith further split from Ismāʿīlīsm as it developed into a distinct monotheistic Abrahamic religion and ethno-religious group with its own unique doctrines, and finally separated from both Ismāʿīlīsm and Islam altogether. Thus, the Druze do not identify themselves as Muslims, and are not considered as such by Muslims either.

The second split occurred between Nizārī and Musta‘lī Ismāʿīlīs following the death of Ma'ad al-Mustansir Billah in 1094 CE. His rule was the longest of any caliph in any Islamic empire. Upon his death, his sons, Nizār (the older) and Al-Musta‘lī (the younger), fought for political and spiritual control of the dynasty. Nizār was defeated and jailed, but according to the Nizārī tradition his son escaped to Alamut, where the Iranian Ismāʿīlī had accepted his claim. From here on, the Nizārī Ismāʿīlī community has continued with a present, living Imam.

The Musta‘lī Ismāʿīlīs split between the Ṭayyibi and the Ḥāfiẓi; Ṭayyibi Ismāʿīlīs, also known as "Bohras", are further divided between Dawudi Bohras, Sulaymani Bohras, and Alavi Bohras. The former denomination claims that At-Tayyib Abi l-Qasim, son of Al-Amir bi-Ahkami l-Lah, and the Imams following him went into a period of anonymity (Dawr-e-Satr) and appointed a Da'i al-Mutlaq ("Unrestricted Missionary") to guide the community, in a similar manner as the Ismāʿīlīs had lived after the death of Muhammad ibn Imam Ismāʿīl. The latter denomination claims that the ruling Fatimid caliph was the Imam, and they died out with the fall of the Fatimid Empire.

Ismāʿīlīs have categorized their practices which are known as seven pillars:

Walayah (Guardianship) Taharah (Purity)

Salat (Prayer) Zakāt (Charity)

Sawm (Fasting) Hajj (Pilgrimage)

Jihad (Struggle)

The Nizārīs place importance on a scholarly institution because of the existence of a present Imam. The Imam of the Age defines the jurisprudence, and his guidance may differ with Imams previous to him because of different times and circumstances. For Nizārī Ismāʿīlīs, the current Imam is Karim al-Husayni Aga Khan IV. The Nizārī line of Imams has continued to this day as an uninterrupted chain.

Divine leadership has continued in the Bohra branch through the institution of the "Missionary" (Da'i). According to the Bohra tradition, before the last Imam, At-Tayyib Abi l-Qasim, went into seclusion, his father, the 20th Al-Amir bi-Ahkami l-Lah, had instructed Al-Hurra Al-Malika the Malika (Queen consort) in Yemen to appoint a vicegerent after the seclusion—the Da'i al-Mutlaq ("Unrestricted Missionary"), who as the Imam's vicegerent has full authority to govern the community in all matters both spiritual and temporal while the lineage of Musta‘lī-Ṭayyibi Imams remains in seclusion (Dawr-e-Satr). The three branches of Musta‘lī Ismāʿīlīs (Dawudi Bohras, Sulaymani Bohras, and Alavi Bohras) differ on who the current "Unrestricted Missionary" is.

Zaydism, otherwise known as Zaydiyya or as Zaydī Shīʿism, is a branch of Shia Islam named after Zayd ibn ʿAlī. Followers of the Zaydī school of jurisprudence are called Zaydīs or occasionally Fivers. However, there is also a group called Zaydī Wāsiṭīs who are Twelvers (see below). Zaydīs constitute roughly 42–47% of the population of Yemen.

The Zaydīs, Twelvers, and Ismāʿīlīs all recognize the same first four Imams; however, the Zaydīs consider Zayd ibn ʿAlī as the 5th Imam. After the time of Zayd ibn ʿAlī, the Zaydīs believed that any descendant (Sayyid) of Ḥasan ibn ʿAlī or Ḥusayn ibn ʿAlī could become the next Imam, after fulfilling certain conditions. Other well-known Zaydī Imams in history were Yahya ibn Zayd, Muhammad al-Nafs al-Zakiyya, and Ibrahim ibn Abdullah.

The Zaydī doctrine of Imamah does not presuppose the infallibility of the Imam, nor the belief that the Imams are supposed to receive divine guidance. Moreover, Zaydīs do not believe that the Imamate must pass from father to son but believe it can be held by any Sayyid descended from either Ḥasan ibn ʿAlī or Ḥusayn ibn ʿAlī (as was the case after the death of the former). Historically, Zaydīs held that Zayd ibn ʿAlī was the rightful successor of the 4th Imam since he led a rebellion against the Umayyads in protest of their tyranny and corruption. Muhammad al-Baqir did not engage in political action, and the followers of Zayd ibn ʿAlī maintained that a true Imam must fight against corrupt rulers.

In matters of Islamic jurisprudence, Zaydīs follow the teachings of Zayd ibn ʿAlī, which are documented in his book Majmu'l Fiqh (in Arabic: مجموع الفِقه). Al-Ḥādī ila'l-Ḥaqq Yaḥyā, the first Zaydī Imam and founder of the Zaydī State in Yemen, is regarded as the codifier of Zaydī jurisprudence, and as such most Zaydī Shias today are known as Hadawis.

The Idrisids (Arabic: الأدارسة) were Arab Zaydī Shias whose dynasty, named after its first sultan, Idris I, ruled in the western Maghreb from 788 to 985 CE. Another Zaydī State was established in the region of Gilan, Deylaman, and Tabaristan (northern Iran) in 864 CE by the Alavids; it lasted until the death of its leader at the hand of the Samanids in 928 CE. Roughly forty years later, the Zaydī State was revived in Gilan and survived under Hasanid leaders until 1126 CE. Afterwards, from the 12th to 13th centuries, the Zaydī Shias of Deylaman, Gilan, and Tabaristan then acknowledged the Zaydī Imams of Yemen or rival Zaydī Imams within Iran.

The Buyids were initially Zaydī Shias, as were the Banu Ukhaidhir rulers of al-Yamama in the 9th and 10th centuries. The leader of the Zaydī community took the title of caliph; thus, the ruler of Yemen was known by this title. Al-Hadi Yahya bin al-Hussain bin al-Qasim ar-Rassi, a descendant of Ḥasan ibn ʿAlī, founded the Zaydī Imamate at Sa'dah in 893–897 CE, and the Rassid dynasty continued to rule over Yemen until the middle of the 20th century, when the republican revolution of 1962 deposed the last Zaydī Imam. (See: Arab Cold War). The founding Zaydī branch in Yemen was the Jarudiyya; however, with increasing interaction with the Ḥanafī and Shāfiʿī schools of Sunnī jurisprudence, there was a shift from the Jarudiyya group to the Sulaimaniyya, Tabiriyya, Butriyya, and Salihiyya. Zaydī Shias form the second dominant religious group in Yemen. Currently, they constitute about 40–45% of the population in Yemen; Jaʿfaris and Ismāʿīlīs constitute the 2–5%. In Saudi Arabia, it is estimated that there are over 1 million Zaydī Shias, primarily based in the western provinces.

Currently, the most prominent Zaydī political movement is the Houthi movement in Yemen, known by the name of Shabab al-Mu'mineen ("Believing Youth") or Ansar Allah ("Partisans of God"). In 2014–2015, Houthis took over the Yemeni government in Sana'a, which led to the fall of the Saudi Arabian-backed government of Abd Rabbuh Mansur Hadi. Houthis and their allies gained control of a significant part of Yemen's territory, and resisted the Saudi Arabian-led intervention in Yemen seeking to restore Hadi in power. (See: Iran–Saudi Arabia proxy conflict). Both the Houthis and the Saudi Arabian-led coalition were being attacked by the Sunnī Islamist militant group and Salafi-jihadist terrorist organization ISIL/ISIS/IS/Daesh.

The history of Shia–Sunnī relations has often involved religious discrimination, persecution, and violence, dating back to the earliest development of the two competing sects. At various times throughout the history of Islam, Shia groups and minorities have faced persecution perpetrated by Sunnī Muslims.

Militarily established and holding control over the Umayyad government, many Sunnī rulers perceived the Shias as a threat—both to their political and religious authority. The Sunnī rulers under the Umayyad dynasty sought to marginalize the Shia minority, and later the Abbasids turned on their Shia allies and imprisoned, persecuted, and killed them. The persecution of Shia Muslims throughout history by their Sunnī co-religionists has often been characterized by brutal and genocidal acts. Comprising only about 10–15% of the global Muslim population, Shia Muslims remain a marginalized community to this day in many Sunnī-dominant Arab countries, without the rights to practice their religion and freely organize.

In 1514, the Ottoman sultan Selim I (1512–1520) ordered the massacre of 40,000 Alevis and Bektashi (Anatolian Shia Muslims). According to Jalal Al-e-Ahmad, "Sultan Selim I carried things so far that he announced that the killing of one Shia had as much otherworldly reward as killing 70 Christians." In 1802, the Al Saud-Wahhabi armies of the Ikhwan from the First Saudi State (1727–1818) attacked and sacked the city of Karbala, the Shia shrine in Najaf (eastern region of Iraq) that commemorates the martyrdom and death of Ḥusayn ibn ʿAlī.

During the rule of Saddam Hussein's Ba'athist Iraq, Shia political activists were arrested, tortured, expelled or killed, as part of a crackdown launched after an assassination attempt against Iraq's Deputy Prime Minister Tariq Aziz in 1980. In March 2011, the Malaysian government declared Shia Islam a "deviant" sect and banned Shia Muslims from promoting their faith to other Muslims, but left them free to practice it themselves privately.

The most recent campaign of anti-Shia oppression was the Islamic State organization's persecution of Shias in its territories in Northern Iraq, which occurred alongside the persecution of various religious groups and the genocide of Yazidis by the same organization during the Second Iraq War.

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