The Russo-Ukrainian War has seen foreign volunteers participate on both sides of the conflict. Most foreign fighters joined the conflict during one of two waves. The first wave happened from 2014 to 2019 during the War in the Donbas and consisted of approximately 17,241 foreign fighters. The second wave is considered by researchers to have been much larger and began in 2022 after the Russian invasion of Ukraine.

The Russo-Ukraine War has seen two distinct waves of foreign fighters: the 2014 wave to join Ukrainian volunteer battalions and pro-Russia separatist groups during the War in the Donbas phase and the post-Russian Invasion of Ukraine wave starting in 2022.

The foreign fighter movement in 2014 was largely short-lived, with researcher Kacper Rekawek writing, "fighters arrived throughout the summer of 2014, and most of them were gone from Ukraine at some point in 2015, although some returned later, with a small group settling in Ukraine permanently." By the end of 2015, Rekawek notes, "both sides took steps to professionalise their forces and incorporate the bottom-up organised volunteer battalions into e.g. the Ukrainian National Guard or, in the case of the 'separatists,' into the 'army corps.' This effectively meant an end to foreign fighter recruitment for this conflict and very few (new) foreigners joined either side after the end of 2015."

An analysis of foreign fighters by Arkadiusz Legieć, a Senior Analyst at the Polish Institute of International Affairs, estimated that about 17,241 foreign fighters fought in Ukraine between 2014 and 2019. 3,879 of those foreign fighters supported Ukraine and joined foreign volunteer battalions. The largest group of foreign fighters in Ukraine was approximately 3,000 Russian citizen volunteers. The second-largest group consisted of approximately 300 Belarusians. The third-largest group consisted of approximately 120 Georgians. The only other country to exceed 50 foreign fighters was Croatia, with approximately 60 fighters. Other countries whose nationals supported Ukraine included Albania (15), Australia (5), Austria (35), Azerbaijan (20), Belgium (1), Bosnia and Herzegovina (5), Bulgaria (6), Canada (10), Czech Republic (5), Denmark (15), Estonia (10), Finland (15), France (15), Germany (15), Greece (2), Ireland (7), Israel (15), Italy (35), Latvia (8), Lithuania (15), Moldova (15), Kosovo (4), Netherlands (3), North Macedonia (4), Norway (10), Poland (10), Portugal (1), Romania (4), Serbia (6), Slovakia (8), Sweden (25), Turkey (30), the United Kingdom (10), and the United States (15).

The launch of the Russian invasion of Ukraine in February 2022 caused a significant increase in the number of foreign fighters in the conflict. The Ukrainian government announced the establishment of an officially-sanctioned foreign legion two days after it began, which had received alleged endorsement from some Western governments after ambiguous statements from officials. The Georgian Legion and Belarusian Kastuś Kalinoŭski Regiment and are one of the largest such units on the Ukrainian side.

Russian president Vladimir Putin publicly welcomed fighters from abroad to join his forces. Most foreigners arriving to fight for Russia are enlisted in the Prizrak Brigade.

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A significant number of foreign fighters from the former Soviet Union have fought in Russo-Ukrainian War for the pro-Russian side. These have included fighters from Armenia, Belarus, Kazakhstan, Kyrgyzstan, and Latvia, as well as from pro-Russian breakaway regions such as Abkhazia, South Ossetia and Transnistria. Some have fought as volunteers due to being members of Russian diaspora or ideological reasons, while others are mercenaries allegedly hired by the Russian government.

Prior to its open involvement in 2022, especially during the first peak of the war in Donbas between 2014 and 2015, Russia had previously tried to deny any formal intervention in Ukraine and portrayed Russian forces in the country as either part of local forces or Russian citizens voluntarily fighting in the country. These units have been referred as "little green men" by Western and Ukrainian media and as "polite people" by Russian media. The Russian government-supported private military company Wagner Group, largely composed of former Soviet, Eastern European and Syrian soldiers, is deployed in Ukraine to secure Russian interests while maintaining the deniability of formal Russian involvement.

Outside of Russian forces which have been described as volunteers or unrelated to the Russian government to maintain deniability, various Russian individuals and groups have joined separatist forces in Donbas. These have included Cossacks, the pro-Putin National Liberation Movement, the neo-Nazi Russian National Unity, the National Bolshevik Other Russia party and left-wing activists. Non-Slavic ethnic minorities have been among the foreign fighters, including North Ossetians, Ingush, and Chechens.

Most foreign fighters from countries outside the former Soviet Union are from Europe. According to a report by Polish security expert Kacper Rękawek for the Polish Institute of International Affairs published in 2015, around a hundred Germans, a hundred Serbs, and thirty Hungarians are fighting for pro-Russian forces in Donbas.

In December 2023, the government of Nepal revealed that six of its citizens had been killed in Ukraine fighting for Russian forces and urged Russia to stop recruiting young men from the country to fight in its army. Nepal's ambassador to Russia claimed around 150-200 Nepali nationals have been fighting for Russia. In early January 2024, Nepal stopped issuing permits for its citizens to work in Russia or Ukraine after 10 of its citizens were killed fighting in Russia's army. The number of Nepalis killed in Ukraine is officially 12 but could be as high as 19.

In addition, Nepali foreign minister Narayan Prakash Saud said that as of December 2023, four Nepali men had been taken prisoner by the Ukrainian army, and about 100 were missing or injured.

A number of Indian nationals were reportedly 'duped' into fighting in Russia's army after being promised other jobs, admissions to "dubious private universities" and "free discounted visa extensions" within Russia, but were instead sent to the front lines. The identities of at least two Indian nationals expecting to work as "helpers" have been confirmed killed fighting at the front in Ukraine. In March 2024, India said it had uncovered a "major human trafficking network" which lured young men to Russia with the promise of jobs only to force them to fight in the war.

A significant number of Serbian citizens and ethnic Serbs from neighbouring countries such as Bosnia (specifically the autonomous Republika Srpska) and Montenegro have joined to fight for pro-Russian forces in Donbas, having been described by external observers and the DPR/LPR authorities as one of the largest components of foreign fighters. Many of the Serb fighters are veterans of the Yugoslav Wars. Serbian units within the pro-Russian forces include the International Brigade, Seventh Brigade, Serbian-Hussar Regiment, Ural Unit, First Slavic Unit, Batman Unit, Rezanj Unit and the Jovan Šević Detachment.

Historical links with Russia, pan-Slavism and religious affinity have been regarded as a major factor in Serbs joining the pro-Russian forces, although many are mercenaries. Many of the fighters identify as Chetniks. In January 2024, Serbian DPR fighter Dejan Berich spoke of abuse of Serbian recruits with Russian paratroopers referring to the Serbs as ″gypsies″ and sending them on assaults with two or three magazines of ammunition, ordering them to obtain weapons in the battlefield.

Left-wing volunteers have gone to fight for the pro-Russian forces, accusing the Ukrainian government of being a "fascist state" and seeking to engage in an "anti-fascist struggle". However, these leftist volunteers have co-operated with far-right groups in Donbas. Among the initial volunteers were members of the Communist Party of Ukraine, as well as some members of trade unions and labor organizations opposed to the new government that emerged after the Euromaidan Ukrainian Revolution.

A small number of Spanish socialists travelled to Ukraine to fight for the separatists, with some explaining they were "repaying the favour" to Russia for the USSR's support to Republicans during the Spanish Civil War. They were also enlisting in solidarity with those who died in the Unions House fire. Spanish fighters established the 'Carlos Palomino International Brigade', which fought under the flag of the Second Spanish Republic. In 2015, it reportedly had less than ten members. Other examples include the 'DKO' (Volunteer Communist Unit) and the Interunit, both composed of foreign communist volunteers.

Latvian communist of Ugandan and Russian descent Beness Aijo was arrested in Donetsk in 2014 for fighting with separatist forces and the National Bolshevik Interbrigades. A female member of the Israeli Communist Party had also reportedly gone to fight for the separatists in 2015.

Far-right foreign fighters from Europe and to a lesser extent North America have fought alongside the pro-Russian separatists in Donbas, including white nationalists, neo-Nazis, fascists and Christian extremists. Motivations for these fighters have included the belief that they are fighting America and Western interests and that Vladimir Putin is a bulwark for traditional white European values who they must support against the decadent West.

One notable far-right group is the French organisation Continental Unity, which has been accused of recruiting far-right extremists across Europe to come and fight in Donbas. Other far-right groups include the Bulgarian nationalist Orthodox Dawn Battalion and the Hungarian nationalist Legion of Saint Istvan.

Finnish far-right and neo-Nazi groups have been recruited for the war in Ukraine by local far-right pro-Russian parties.

The Ukrainian conflict has shattered the Italian far right and while some CasaPound Italia militants are volunteers in the Ukrainian ranks alongside the Azov battalion with which they have a long-standing collaboration, other radical far-right militants are aligned with the pro-Russian separatist militias of Donbas. In particular, the latter fight together with volunteers with a far-left political background

Following its 2022 offensive, US and Ukrainian intelligence have alleged that Russia has sought to hire and already deployed fighters from forces it supports in places such as Syria, Libya and the Central African Republic under the command of the Wagner Group private military forces. Fighters have reportedly included those from the pro-Syrian government National Defence Forces, Central African Union of Peace and the pro-Khalifa Haftar Libyan National Army. The Libyan National Army denied any of its fighters were fighting for Russia in Ukraine.

The Iran-aligned Lebanese political party Hezbollah denied sending fighters to Ukraine after the General Staff of the Ukrainian Armed Forces accused some of their members of being among 1,000 Syrian mercenaries allegedly hired to fight in Ukraine.

Turkish media and Russian-American journalist Vera Mironova alleged that members of the Kurdistan Workers' Party and the People's Defense Units (both affiliated with the Kurdistan Communities Union) were fighting as mercenaries for Russian forces.

On 4 October, the Syrian Observatory for Human Rights reported that 5 Syrian Arab Army soldiers of the 25th Special Mission Forces Division had been killed fighting for Russia during the 2022 Ukrainian southern counteroffensive.

In January 2023, the Syrian Observatory for Human Rights reported that just under 2000 soldiers of the Syrian Arab Army, specifically the 25th Special Mission Forces Division, had been deployed to fight on behalf of Russia in Ukraine. The SOHR had obtained a document allegedly from the Syrian Government, explaining the Syrian Army's budget and salaries affecting Syrian soldiers in Ukraine.

It was widely reported in October 2022 that the Wagner Group had attempted to recruit former members of the American-trained Afghan National Army Commando Corps which became defunct after the victory of the Taliban insurgency in August 2021.

In June 2024, Bloomberg reported that Russia was detaining and forcing Africans on work visas to decide between deportation or fighting in the war.

On 23 July 2024, Tokyo reported the death of a Japanese national, an ex-JGSDF soldier who was last stationed in Osaka, was reportedly killed on 3 June.

Cuban mercenaries are reported to have participated in the conflict.

A number of African students or former students are known to have been recruited for military service on the Russian side. Jean-Claude Sangwa, a student from the Democratic Republic of the Congo (DRC) who had formerly studied in Rostov, was reported to have enlisted in Luhansk People's Militia in late 2021 alongside two other expatriates from the DRC or Central African Republic.

A number of other African former students are known to have been recruited by the Wagner Group after receiving criminal convictions while in Russia. Lemekani Nathan Nyirenda, a Zambian former student at the Moscow Engineering Physics Institute who had been sentenced to a nine-and-a-half year jail term, was killed while fighting for the Wagner Group in September 2022. Nemes Tarimo, a Tanzanian former student at Moscow Technological University who had previously received a seven-year jail term, was killed in similar circumstances in October 2022.

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Similarly to their opponents, many foreign fighters from the former Soviet Union have fought in the Russo-Ukrainian War for the Ukrainian side. These have included fighters from Armenia, Azerbaijan, the Baltic states, Belarus and Georgia. Exiled Chechens opposed to Ramzan Kadyrov and some Russian dissidents have also taken up arms on behalf of the Ukrainians.

Georgians have been participating in the conflict on the side of Ukraine, since 2014. Large formations such as the Georgian Legion that consist of 1,000 fighters, are mostly made up of Georgian, but also various other nationalities, including British.

Unofficially, since the launch of Russia's invasion in 2022, an estimated 1.500 Georgians are fighting for Ukraine in various Ukrainian Armed Forces units and the International Legion.

Already during the War in Donbas (2014–2022), Belarusians fought alongside Ukraine, forming the Pahonia Detachment and the Tactical group "Belarus".

In spring 2022, it was reported that more than 200 Belarusian citizens have joined the Armed Forces of Ukraine to defend Ukraine from the Russian invasion, with another 300 volunteers from Belarus came through Poland. In July 2023, Radio Free Europe/Radio Liberty reported that around 450 Belarusians fight for Ukraine.

In the beginning of the full-scale invasion, Belarusians have created a separate battalion named after Kastuś Kalinoŭski to defend Kyiv. It later transformed into a regiment consisting of two battalions, u UAV unit and a medical company. The Kastus Kalinouski Regiment was also joined by Pavel Shurmei, a former Belarusian Olympic rower and world record holder.

Another Belarusian volunteer battalion fighting for Ukraine, a part of the Kastuś Kalinoŭski Battalion, was known as the Pahonia Regiment. It ceased to exist in 2023.

Other Belarusian units include the Belarusian Volunteer Corps which includes the Terror Battalion. There are also around 20 Belarusians fighting in the Second International Legion.

In June 2023, Valery Sakhashchyk, effective defence minister of the Belarusian United Transitional Cabinet (a government-in-exile opposed to the de facto government of Alexander Lukashenko) informed about the creation of the 1st separate amphibious assault company "Belarus", which is part of the 79th Air Assault Brigade. The Belarusian assault company is located in the Donetsk direction.

Several other Belarusians are fighting in different Ukrainian military formations as well.

Ajnad al-Kavkaz, a group of Chechen foreign fighters which fights on the side of the Syrian opposition, which is fighting the Russian government in the Syrian civil war, deployed some fighters to fight for the Ukrainian side. Like other Chechen foreign fighter groups across the world, they are motivated by anti-Russian sentiment stemming from the Chechnyan Conflict.

A small number of South Koreans have also volunteered for Ukraine, although they were legally prohibited from doing so, due to concerns that it could cause diplomatic issues for South Korea. Four South Koreans had been convicted for serving in Ukraine by 2023.

Foreign expats living in Ukraine have also fought in the Ukrainian forces. Brahim Saadoun, a Moroccan national, studied in the country as a foreign student from 2019 and enlisted in the Ukrainian army in 2022. He was captured by Russian forces but released to Morocco in September 2022 in an agreement brokered by Saudi Arabia.

Three days after the beginning of the Russian invasion in 2022, Ukrainian President Zelenskyy announced the establishment of the International Legion.

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• Interbrigades
• Pavel Sudoplatov Battalion
• Prizrak Brigade (Unofficially, attracts large numbers of foreign volunteers)
• Vostok Brigade
• Pyatnashka Brigade

• Dzhokhar Dudayev Battalion
• Freedom of Russia Legion
• Georgian National Legion
• International Legion of Territorial Defense of Ukraine
• Kastuś Kalinoŭski Regiment
• Misanthropic Division
• Sheikh Mansur Battalion
• Turan Battalion
• Ajnad al-Kavkaz
• Norman Brigade
• Pahonia Regiment
• Russian Volunteer Corps
• Ichkerian Special Purpose Battalion
• Tactical group "Belarus"
• Russian Insurgent Army

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Pro-Russia

• Beness Aijo, Latvian Nazbol of Russian and Ugandan descent who was arrested and deported from Ukraine for fighting with the Interbrigades in 2014. He later returned to Ukraine to fight for the Donetsk People's Republic.
• Dejan Berić, Serbian sniper in the armed forces of the DNR.
• Russell Bentley, a former American soldier and communist who fought with Russian separatists in 2014 and returned in 2022 with the expressed intention of helping Russian forces. Believed to have been tortured and killed by fellow Russian servicemen mistaking him for a US spy in 2024.

Pro-Ukraine

• Trevor Cadieu, a three star Canadian general, possibly captured in June 2022.
• Sandra Andersen Eira, Norwegian Sámi member of the Sámi Parliament of Norway (2017–2021) who joined the Ukrainian International Legion.
• Aleko Elisashvili, Georgian parliamentarian who joined the Ukrainian International Legion.
• Mamuka Mamulashvili, Georgian paramilitary leader and former soldier who leads the Georgian National Legion.
• Isa Munayev, Chechen rebel who led the Dzhokhar Dudayev Battalion before his death at the Battle of Debaltseve in 2015.
• Malcolm Nance, American author, media pundit, and former US Navy Senior Chief Petty Officer who joined the Ukrainian International Legion.
• Shaun Pinner, former British soldier who joined the Ukrainian Army as a volunteer and was captured and sentenced to death by the authorities of the Donetsk People's Republic.
• Tseng Sheng-guang, Taiwanese indigenous veteran who volunteered in the Ukrainian International Legion and the Carpathian Sich Battalion, was the first East Asian soldier killed in action.
• Pavel Shurmei, Belarusian Olympic rower who joined the pro-Ukrainian Belarusian Kalinoŭski Regiment.
• Aliaksiej Skoblia, Belarusian dissident who defected to Ukraine in 2015 and currently leads the Kalinoŭski Regiment.
• Yulia Tolopa, Russian-born woman fighting on the side of Ukraine from 2014 to 2019

• International Legion (Ukraine)
• Ukrainian volunteer battalions
• Foreign fighters in the Syrian Civil War and War in Iraq

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