The war in Ukraine and international law: Prosecuting the Crime of Aggression

Written by Yuliya Mik



As the first anniversary of Russia’s invasion of Ukraine has just passed, there are few signs of the war coming to an end in the near future. Readers will recall that the conflict between Russia and Ukraine – which has been ongoing since 2014 – took a significant turn on 24 February 2022, with Russia’s military invasion of Ukraine.

Since then, as has been widely reported, Russia has conducted, and continues to launch, numerous attacks against civilians and civilian infrastructure in Ukraine, including ongoing missile strikes targeting residential blocs, hospitals and other civilian objects, and Ukraine’s energy infrastructure. In light of Russia’s ongoing war against Ukraine, the calls to hold Russia and its political and military leaders accountable for violations of international law continue.

This article briefly examines the current state of affairs regarding international law and Russia’s war against Ukraine.

The war in Ukraine and international legal proceedings

The war in Ukraine has given rise to multi-faceted international legal proceedings.

Firstly, in the realm of individual criminal responsibility, the four ‘core international crimes’ (namely, genocide, crimes against humanity, war crimes and the crime of aggression) have all been invoked as the subject of actual or anticipated international legal proceedings, in both domestic and international courts. (More on aggression, below.)

  • Internationally, international criminal proceedings have been initiated by the International Criminal Court (ICC), when on 2 March 2022, the ICC Prosecutor opened an investigation into the Situation in Ukraine, looking into the many allegations of war crimes, crimes against humanity and potentially also genocide on the territory of Ukraine. On 17 March 2023, in a major development in the proceedings, the ICC Pre-Trial Chamber issued arrest warrants against Russian President Vladimir Putin and Russia’s Commissioner for Children’s Rights Maria Lvova-Belova.
  • Domestically, Ukraine has conducted a small number of domestic prosecutions of Russian soldiers for war crimes, and is continuing to conduct investigations into crimes committed during the war, with a view to further prosecutions.

Secondly, in the realm of state responsibility, Ukraine has filed proceedings against Russia invoking various areas of international law.

  • On 26 February 2022, Ukraine filed an application instituting proceedings against the Russian Federation before the International Court of Justice (ICJ). In a key development, on 16 March 2022, the ICJ granted an Order for Provisional Measures, ordering Russia to  “immediately suspend the military operations that it commenced on 24 February 2022 in the territory of Ukraine”. While strictly speaking, the case regards treaty interpretation (specifically, the interpretation of the Genocide Convention, to which both Ukraine and Russia are states-parties), in the broader context, the ICJ’s Order for Provisional Measures is a powerful statement concerning the legality and credibility of Russia’s claims justifying its military invasion.
  • At the same time, Ukraine also filed an application against Russia at the European Court of Human Rights (ECtHR), alleging “massive human rights violations” by Russian troops on the territory of Ukraine. Here, too, the Court took the opportunity to quickly impose urgent Interim Measures, ordering Russia “to refrain from military attacks against civilians and civilian objects”. (Although Russia was expelled from the Council of Europe, and by extension the ECtHR’s jurisdiction, on 16 March 2022, the Court may still hear and rule on pending cases against Russia, including this case.)

Further, and very tellingly, the above legal proceedings have attracted levels of intervention by third-states not seen to date. The ICC Prosecutor has received an unprecedented 43 state-party referrals of the situation – including both Australia and New Zealand. In Ukraine’s application to the ICJ, too, an unprecedented number of third-states have filed third-party interventions in these proceedings (so far 33) – again, including both Australia and New Zealand. And at the ECtHR, likewise, an unprecedented number of states (so far, 25 – more than half of the member-states to the European Convention on Human Rights) have requested leave to intervene as third parties in this case. This level of third-state intervention no doubt reflects the widespread international condemnation of Russia’s aggressive actions.

The war in Ukraine and the crime of aggression

On 2 March 2022, the United Nations General Assembly (UNGA) condemned Russia’s invasion of the Ukraine and called it an act of aggression. This is a key development, including in the realm of international criminal law, as prosecuting the crime of aggression requires proving that an act of aggression has occurred – and while the UNGA’s resolution may not be determinative of the matter, it will certainly be instructive. Indeed, since then, there have been persistent calls for the creation of an international tribunal to individually prosecute senior Russian political and military leaders for the crime of aggression.

Individual criminal prosecutions for the crime of aggression have been extremely rare – indeed, the last such case was at the post-World War II Nuremberg and Tokyo Tribunals. However, the possibility of the next historic prosecution for what was once labelled “the supreme international crime” has gained renewed attention, and discussions in both popular discourse and scholarly circles continue regarding the mechanics and practicalities of how this might be done.

Three models have thus featured most prominently in current discourse regarding prosecuting the crime of aggression: a treaty-based tribunal, a specially established ad hoc tribunal endorsed by the UNGA, and a hybrid internationalised tribunal embedded in the domestic courts of Ukraine. Each of these models faces certain advantages and drawbacks, concerning issues such as immunities from prosecution, support from the international community, the prospects of enforcement, and logistical considerations.

Option 1: A treaty-based tribunal

In this case, the ICC has no jurisdiction over the crime of aggression, owing to the multiple jurisdictional hurdles that must be overcome before the ICC can prosecute this particular crime. However, one option is the creation of a treaty-based tribunal, following the model of the ICC. As the ICC cannot prosecute the crime of aggression in this case, similar models have been proposed, including the creation of a treaty-based court by way of an agreement between Ukraine and some other international organization.

One prominent proposal is an agreement between Ukraine and the European Union (EU), leading to the creation of a regional tribunal for the prosecution of aggression. Supporters of this model argue that the EU is a perfect partner for such a project, given the high level of support that the EU has shown Ukraine. Further, the EU has already shown its international justice credentials with the efforts it has contributed to capacity-building for international justice projects in a number of post-war former Yugoslav states such as Bosnia and Kosovo.

However, one question would be whether it would be appropriate for a regional tribunal to prosecute this international crime in the name of the international community as a whole – especially such a momentous international legal development as prosecuting an international crime for the first time since the Nuremberg and Tokyo Tribunals. 

Option 2: An ad hoc tribunal

Another proposal that has been put forward is for a specially established ad hoc tribunal endorsed by the UNGA. This kind of international tribunal could be created by an agreement between Ukraine and the United Nations, with approval by the UNGA, as was done for example in the cases of the Special Court for Sierra Leone (SCSL) and the Khmer Rouge Tribunal in Cambodia (ECCC). This is indeed the model that Ukraine itself has been calling for.

Proponents of this model point to the fact that unlike other proposals, this kind of tribunal would be truly international in character, and would also have the benefit of international support in the form of international judges and prosecutors, amounting to a model that is truly representative of the international community as a whole. Proponents also argue that this is potentially the only proposed model that would avoid the problem of immunities for heads of state and government, including Russian President Vladimir Putin.

However, critics have argued that this model could attract questions of selective justice and thus suffer from a serious legitimacy deficit: why ought the United Nations create an international tribunal to prosecute aggression against Ukraine, but not, for example, Iraq or Afghanistan?

Option 3: a hybrid or internationalised court

Another proposal has been the creation of a hybrid or internationalised tribunal embedded in the domestic courts of Ukraine, similar to the Kosovo Special Chambers and the Bosnian War Crimes Chamber models.

As noted above, Ukraine is already carrying out criminal investigations and prosecutions of other international crimes in its domestic courts; prosecuting individuals for the crime of aggression against its own territorial integrity would thus not be such a stretch. Ukrainian judges and prosecutors, possibly with the support of international judges and prosecutors, would be applying the internationally accepted definition of international crimes, including the crime of aggression, within Ukraine’s domestic legal system.

Critics of this option, however, point to the potential problem of immunities for heads of state and government before national courts – which remains a fundamental rule of international law – and which would, therefore, leave a manifest gap in any domestic prosecution efforts. Critics also point to the potential legitimacy of these kinds of prosecutions emanating from Ukraine’s domestic courts, which would simply not be able to stand up to that of a truly international court or tribunal. The potential difficulty in executing any arrest warrants issued by the Ukrainian courts is another area of concern in relation to this proposed model.


The calls for an international court or tribunal to prosecute the crime of aggression continue to grow louder. Recently, this notion has been endorsed by the EU, the Council of Europe, the Organization for Security and Co-operation in Europe (OSCE), NATO, The Elders (an international NGO of senior public figures, peace activists and human rights advocates), numerous prominent international legal scholars, as well as a number of individual states such as the UK and Germany. However, there remains disagreement on the best way to go about this.

The war against Ukraine is undoubtedly a matter for international concern, challenging the very rules-based order on which the post-World War II era has been built and sustained. While the enforcement of international law, including carrying out individual criminal prosecutions, may face many challenges, we must not lose sight of the importance of upholding the highest standards of international justice and the rule of law, even in the face of jurisdictional challenges and imperfect institutional solutions. After all, there can be no justice without peace – and no peace without justice.

Yuliya Mik
Yuliya Mik
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Yuliya is a Barrister at the Victorian Bar and a member of Greens List Barristers, practising principally in the areas of domestic criminal law and public international law. Yuliya is currently completing her PhD in international law at the Hebrew University of Jerusalem. Yuliya has lectured for many years in both domestic and international law. Yuliya was associate lecturer at the School of Law, Deakin University, and is currently associate lecturer at the Faculty of Law, the Hebrew University of Jerusalem. Yuliya has published academic articles on various areas of public international law, and regularly lectures, presents at conferences and seminars around the world in all areas of public international law.

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