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Foreign Fighters, Foreign Volunteers and Mercenaries in the Ukrainian Armed Conflict

11 Jul 2022

Shortly after the expanded Russian invasion of Ukraine on 24 February 2022, President Zelensky created the International Legion for the Territorial Defence of Ukraine, and began calling for (foreign) volunteers to join. A special website has been dedicated to assist foreign volunteers to come and support Ukraine. Since the start of the conflict, Ukraine claims that approximately 20,000 individuals have joined the Ukrainian armed forces.

Foreign individuals also joined the Russian side of the conflict. Russian President Vladimir Putin has announced that 16,000 volunteers from the Middle East are ready to support Russia. Foreigners from Chechnya and mercenaries joining the Wagner Group from Syria and Libya are included.

Considering the  death sentence imposed on two British and Moroccan fighters pronounced by a pro-Russian court in East Ukraine, and the recent capture of two American fighters in Donetsk, this perspective addresses the legal status of the foreign individuals joining the conflict in Ukraine, and some of the legal and political consequences of this situation. It provides an overview of some of the salient features of international humanitarian law, offers definitions of the terms ‘foreign volunteer’, ‘foreign terrorist fighter’ and ‘mercenary’, explains why these distinctions matter, and explores what lessons can be drawn from conflating or exchanging these different terms.

Defining the Conflict According to International Humanitarian Law

International Humanitarian Law (IHL) aims to protect civilians and regulate conduct during armed conflict. The classification of a conflict determines which rules are applicable. The current conflict in Ukraine is classified as an international armed conflict (IAC) and is governed by the four Geneva Conventions (GCs), reflecting customary international law, and Additional Protocol I (API), which both Ukraine and Russia are parties to.

IACs are armed conflicts typically between two or more states. In the current context, an IAC began with the Russian military occupation of Ukraine’s Crimea in 2014, which continues today. Simultaneously, a non-international armed conflict (NIAC) erupted in the separatist provinces Donetsk and Luhansk in eastern Ukraine, which claim to be independent republics. Under IHL, a NIAC requires sufficiently intense military violence between, for example, a state and an organised non-state armed group. Prior to Russia’s further intervention in 2022, approximately 17,000 fighters from fifty-five countries, predominantly from Russia, had already joined the conflict in Ukraine since 2014, supporting either the pro-Russian separatists or the Ukrainian side. The existence of a NIAC means that common article 3 of the GCs and Additional Protocol II (APII), also ratified by Ukraine and Russia, are applicable. However, some argue that since the recognition, invasion and if Russia exercises overall control of separatist forces, the conflict would be considered internationalised and an IAC. Other experts believe that armed groups in eastern Ukraine are sufficiently independent to maintain the qualification as a NIAC.

During an armed conflict not all acts of violence are prohibited, whereas generally, under terrorism laws all acts of violence are prohibited. The principle of distinction under IHL determines that combatants and military objectives can be targeted, whereas civilians and civilian objectives are protected. Armed forces of both parties to an IAC are authorised to use force during an armed conflict and cannot be prosecuted for their participation in hostilities, provided the use of force is in conformity with IHL. In contrast, civilians can be criminally prosecuted for directly participating in hostilities during the conflict.

Several terrorism conventions contain an exclusion clause, excluding the application of the convention to certain acts occurring during an armed conflict, which are then solely governed by IHL and other applicable international laws. The standard exclusion clause contained in six conventions relating to terrorist bombings, nuclear terrorism, radioactive material, and aviation and maritime safety excludes all activities of armed forces during an armed conflict. It is understood to thus exclude activities of non-state armed groups during a NIAC.

However, the way states implement the exclusion clause in their domestic legislation differs greatly. Some exclude specific-offences, others only exclude activities that are compliant with IHL from counter-terrorism laws, and yet other countries do not implement the exclusion clause at all, leading to the full application of both IHL and terrorism laws.

Combatants and Prisoner of War Status

In IACs, combatants are broadly defined as those in the armed forces of a party to the conflict. Specifically, armed forces are ‘all organised armed forces, groups or units which are under a command responsible to that party for the conduct of its subordinates, even if that party is represented by a government or an authority not recognised by an adverse party.’ Combatants have the right to directly participate in hostilities and, per combatant’s privilege, are afforded Prisoner of War (POW) status if captured or having fallen into enemy hands during an IAC. In a NIAC, however, members of non-state armed groups are not granted combatant or POW status. Their participation in hostilities is not prohibited under IHL itself, but national law may criminalise such conduct under political crimes, such as rebellion or insurgency.

GC (III) regulates the rights of POWs and stipulates, under Article 4, that POW status is granted to members of: armed forces, other militias, other volunteer corps, as well as civilians involved in a levee en masse. The provisions in GC (III) establish the basic rules and obligations for Detaining Powers, aim to prevent further participation in hostilities, and to ensure that all POWs must be treated humanely. The UN Human Rights Monitoring Mission in Ukraine reported both sides of the conflict have allegedly abused POWs through interrogation and intimidation that could constitute a war crime.

Foreign volunteers and foreign fighters

After the self-proclaimed independence of Donetsk and Luhansk in 2014, a large number of foreign fighters travelled to Ukraine to join the non-state armed groups or state forces. While many volunteers enlisted in the armed forces in Ukraine, others formed volunteer battalions. By 2015, Ukraine incorporated most of the volunteer brigades, and the foreign fighters they contained, into the Ukrainian Army. Additionally, Ukraine adopted the law on amending certain legislative acts of Ukraine regarding foreigners and stateless persons serving in the armed forces of Ukraine allowing stateless persons and foreigners who legally reside in Ukraine to voluntarily join the armed forces. Subsequently, any foreign individuals now integrated into the Ukrainian armed forces are combatants and entitled to POW status. Foreigners that support pro-Russian separatists in eastern Ukraine are classed as foreign fighters, whereas foreigners joining the Russian armed forces are combatants.

Volunteers joining a foreign army may be prosecuted if it is considered an offence in their home country. For example, in the United Kingdom, it is generally not permitted to join a foreign army. In the Netherlands, a citizen is allowed to join a foreign army, except when it concerns a foreign army with whom the Netherlands is, or is likely to be, at war with. In Canada, home to the second largest Ukrainian diaspora, it is not permitted to join an army that is at war with a friendly nation, which means that Canadians may not join the Russian Army but could join the Ukrainian Army. In Australia, one may join a foreign army (not at war with Australia), however a foreign country may not be allowed to openly recruit for volunteers and joining non-state forces is a crime.

Even if individuals are permitted by law to join a foreign army, those who join the Ukrainian or Russian armed forces could face prosecution for terrorist offences, because no exclusion clause is implemented in their country, therefore both IHL and terrorism legislation apply. As noted, examples of this are Australia and the Netherlands whereby individuals may lawfully enlist in a foreign army, but still be prosecuted for terrorist-related offences.

Foreign ‘terrorist’ fighters

The mobilisation of foreign fighters in conflicts is not new, dating back to 1816. More recently, foreign fighters joined the Mujahideen in Afghanistan in the eighties after the Russian invasion. Foreign fighters have also joined the conflicts in Bosnia, Chechnya and, albeit in smaller numbers, in Somalia. The turning point was when 40,000 persons from more than 110 countries travelled to join the Islamic State in Syria and Iraq from 2014 onwards. This drew the attention of the international community, and in 2014 the UN Security Council (UNSC) adopted the landmark resolution 2178 imposing binding obligations on states to stem the flow of foreign ‘terrorist’ fighters.

This resolution has been criticised for various reasons, amongst which is the alleged lack of legislative powers of the UNSC to impose binding obligations that require Member States to criminalise such a wide range of activities related to foreign fighters, such as providing or receiving training, membership of a terrorist organisation, or travelling to a conflict zone. More concerning is the lack of clear definition of the term foreign terrorist fighter. The resolution itself blurs the distinction between terrorism and participation in an armed conflict that may be lawful under IHL, which can be seen now in the conflict in Ukraine.

Many states designate non-state armed groups as terrorist groups and prosecute them under strict terrorism legislation, which often lacks any exceptions for armed conflict. The absence of clear definitions of terrorism enables oppressive regimes to label political opposition or minority groups engaged in non-violent or violent acts as terrorists, and to apply an excessively securitised counter-terrorism response. Examples include the unlawful detention of Uighurs in China, and the stifling of political dissidents in Turkey.

Over-classification of a conflict whereby states respond by applying a military instead of law enforcement framework also occurs, for example the US war on terror against AL-Qaeda globally. This allows states to apply IHL prematurely, when there is legally no conflict, and allows for certain acts of violence, including the use of force against combatants, non-state armed groups, and those directly participating in hostilities. Such violence would not be permitted under the law enforcement approach, which is applicable during peacetime. Yet, under-classification of armed conflict is far more common. For example, Turkey has denied the existence of a NIAC with the Kurdistan Worker’s Party (Partiya Karkerên Kurdistan, PKK), just like Russia previously denied the armed conflict in Chechnya and qualified this as a counter-terrorism operation. We see the same happening now in Ukraine, as Russia justifies the invasion in Ukraine as a special military operation.


According to Article 47 of API  a mercenary is defined as someone who cumulatively fulfils six criteria, namely: 1) being recruited locally or from abroad to fight in an armed conflict; 2) directly participates in hostilities; 3) is motivated by the desire for private gain whereby a party to the conflict promises material compensation in excess of that paid to combatants or armed forces of similar ranks; 4) is neither a national of a party to the conflict nor a resident of a territory controlled by one; 5) is not a member of the armed forces of a party to the conflict; and 6) has not been sent on official duty by another state which is not a party to the conflict.

The use of mercenaries is not prohibited in IACs under API, but being a mercenary deprives a person of combatant and POW status. Mercenaries are therefore civilians and can only be targeted during their direct participation in hostilities. In practice, it can be difficult to prove that someone is a mercenary, particularly in proving that a person is being paid, and even more so being paid more than regular combatants.

Mercenaries and foreign fighters have many similarities: they often have a foreign nationality and  can prolong and/or complicate the conflict. Furthermore, they can both be engaged in terrorist-related activities, organised crimes, and have notably been involved in serious human rights violations. However, while foreign fighters join conflicts for ideological reasons, it is widely understood that mercenaries join conflicts primarily for financial gain. Nonetheless the motives for joining a conflict as mercenary or foreign fighter can be a combination of many factors, making it difficult to separate them purely on this basis.

Russia’s use of mercenaries, particularly the Wagner Group, in the Ukrainian conflict puts these terms to the test. The presence of the Wagner Group supporting pro-Russian armed groups in eastern Ukraine dates back to 2014, and the group is also known to be active in other countries such as Mozambique, the Central African Republic, Libya and Mali. In March 2021, several UN Working Groups and Special Rapporteurs concluded that the Wagner Group had systematically committed human rights violations in CAR, such as sexual- and gender-based violence, summary executions, arbitrary detentions, looting, enforced disappearances and torture during interrogations. Under international human rights law, a state remains responsible for violation by such a group acting for the state.

During a country visit to Ukraine in 2016, the UN Working Group on Mercenaries recognised that although the presence of mercenaries in the conflict in eastern Ukraine was very likely, it remained difficult to distinguish mercenaries from foreign fighters in the conflict. Considering the many human rights violations committed by the Wagner Group mercenaries, it has prompted an international response to their use in conflicts. The European Parliament adopted a resolution in 2021 underlining that their use in Ukraine may aim to disguise military interference in other countries but also avoid or minimise casualties among the Russian armed forces. On 21 April 2022, the EU imposed an asset freeze and travel ban on the unofficial head of the Wagner Group.

While the definition of mercenary is difficult to meet, criminalising mercenary activities is a greater challenge. The International Convention Against the Recruitment, Use, Financing and Training of Mercenaries, only ratified by 37 countries, including Ukraine, requires state parties to criminalise mercenaries. Ukraine and Russia have criminalised the offence of both recruiting and being a mercenary. Despite this, no notable prosecutions have taken place in Ukraine for the act of mercenaryism or for serious violations committed by foreign individuals on both sides of the conflict in eastern Ukraine. It appears that Ukraine has pursued prosecutions for other crimes, including terrorism-related crimes, instead. For example, at the end of May, two Belarus mercenaries allegedly working for the Wagner Group were charged for murdering civilians and for war crimes, in what would be the first instance of mercenaries being held accountable for serious human right violations.

Political and Legal Implications

In mid-April 2022, three men – two UK citizens, Aiden Aslin and Shaun Pinner, along with a Moroccan national – who had been serving with the 36th  Ukrainian Marine Brigade, surrendered alongside their unit to Russian troops in Mariupol. In images circulating on pro-Russian Telegram channels shortly afterwards, Aslin visibly appeared beaten, with further evidence proving his injuries had most likely occurred during his custody in violation of Article 13 of the GC (III), stating that POWs must be humanely treated and protected from acts of violence.

Russia, as the Detaining Power, has certain obligations vis-a-vis POWs, including that they be transferred to another state that is party to the Geneva Conventions. However, these men were subsequently transferred to Donetsk, which is only recognised by Russia as an independent state.

The Supreme Court of the self-proclaimed Donetsk republic has convicted the three foreign volunteers to death for taking part in the hostilities, including the crime of mercenaryism under article 430 of the Donetsk Criminal Code. This has been widely regarded a show trial, violating several rights both under IHL and international human rights law, particularly a POW is entitled to a fair trial by an independent and impartial court applying due process. As explained in another perspective, it appears that IHL does not explicitly prohibit non-state armed groups, such as the Russian backed separatists, to establish courts, but does appear to regulate and provide certain minimum judicial guarantees that need to be met. Therefore, the courts in the self-proclaimed Donetsk republic need to be able dispense justice independently and impartially. Even though non-state armed group cannot adhere to international (human rights) conventions, they need to respect human rights in areas they control and govern. Prior to the 2022 war in Ukraine, the UN Office for Human Rights (UNOHR) already observed that the administration of justice in the self-proclaimed Donetsk republic was flawed and infringes upon the right to fair trial, including the right to a public hearing, to a defence counsel, time to prepare defence and not  be compelled to testify. The three foreign volunteers have been tried in summary proceedings which cannot be considered a fair trial. In fact, by wilfully depriving a POW the right to a fair and regular trial, Russia itself is committing a war crime. In response, the European Court of Human Rights granted interim measures and called upon Russia to ensure that the death penalty will not be executed and to respect the human rights of the foreign volunteers. These include the right to life, the right to be free from torture, and appropriate detention conditions.

Joining a foreign army is risky and once captured, like the UK volunteers, individuals can be mistreated and/or denied POW status (if applicable), making it difficult to return them and creating leverage for Russia while negotiating their release. Foreign volunteers can, upon return, also be prosecuted either for joining a foreign army or for terrorist-related offences. Additionally, there are broader concerns with foreigners joining the conflict in Ukraine, including the potential for drawing more far-right extremists to join the conflict, although the number is lower, and many joining are highly trained veterans. Other concerns are the likelihood of an insurgency or lack of training and discipline of foreign fighters to comply with IHL.

These cases demonstrate why volunteers travelling to Ukraine present foreign governments with diplomatic difficulties, particularly when their citizens are captured by Russian forces. The Russian Defence Ministry has declared those travelling to fight in Ukraine will be regarded as mercenaries, denying foreign volunteers combatant and POW status and increasing the risk of mistreatment. The two British prisoners have also been instrumentalised as hostage diplomacy bargaining chips, with Russian state TV airing clips of the men asking to be exchanged for captive pro-Putin Ukrainian politician Viktor Medvedchuk. An interview with one of the American soldiers, Alexander Drueke, has also been aired on Russian state TV.

If the International Legion became no longer under the responsible command of the Ukrainian army due to heavy losses, the situation would change and its members would no longer be entitled to combatant and POW status, but would become civilians taking a direct part in hostilities. When a pro-government militia groups such as the Ukrainian Aidar battalion do not have an effective command, but with links to the army,  it is highly likely this would lead to a high number of civilian casualties and accountability problems. The Aidar battalion has been involved in widespread abuses, including abductions, unlawful detention, torture, robbery and possible executions, which may amount to war crimes.

Furthermore, where foreign fighters are present this leads to more sexual violence, and that when foreign fighters are not structurally and culturally embedded are more prone to resort to violence against the local population, leading to higher civilian casualties.


If states are not sending armed forces to assist Ukraine, they certainly should not encourage their citizens to join the conflict. Once captured by pro-Russian separatists or Russian forces, individuals face an uncertain fate and their capture could be used for propaganda purposes to demonstrate that the West is involved in the conflict in Ukraine. Furthermore, it could be used to create leverage in negotiating their release. Civilian casualties may increase when foreigners are involved in the conflict and may not only complicate but also could prolong the conflict.

The IHL framework provides a concrete foundation through which fighters are classified during an armed conflict. However, through mislabelling fighters as mercenaries, foreign volunteers or foreign terrorist fighters, individuals are subsequently not afforded the rights or protections they should under the applicable law. The misuse/misunderstanding of terrorism legislation and lack of distinctions between foreign volunteers, foreign fighters and mercenaries undermines the purpose of IHL, to regulate an armed conflict, to protect civilians, and runs the risk of subjecting individuals to poor or inhumane treatment despite them being entitled to POW status, as seen in the Ukrainian conflict.