The International Criminal Court’s arrest warrant against Russian President Vladimir Putin on Friday is just one strand in a complex web of international and national legal moves over alleged war crimes in Ukraine.

Russia has, however, repeatedly denied that its forces have committed atrocities or attacked civilians.

The ICC’s role

The Hague-based tribunal has led the most high-profile investigations into the most prominent suspects, looking into war crimes as well as broader crimes against humanity and genocide.

Since his investigation was launched a year ago, ICC prosecutor Karim Khan has visited Ukraine four times.

He has visited the Kyiv region, where civilians were massacred in Bucha, and the Kharkiv region, home to residential neighbourhoods in the town of Borodianka devastated by shelling, as well as a home for children in southern Ukraine.

The arrest warrants for Putin and Maria Lvova-Belova, Russia’s Commissioner for Children’s Rights, theoretically mark the first step towards an eventual trial – though under current conditions, the capture and arraignment of Russia’s president is almost inconceivable.

Even if that did happen, previous ICC cases have shown it is hard to convict the most senior officials. In more than 20 years, the court has only issued five convictions for core crimes, and none were top officials.

But the ICC investigations into international figures are not the only option. War crimes can also be prosecuted in Ukraine’s own courts, as well as a growing number of countries conducting their own investigations.

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There are also plans to create a new tribunal to prosecute the Russian invasion – which Moscow calls a “special military operation” – as a the crime of aggression. The ICC cannot bring such a charge due to legal constraints.

Who is investigating war crimes on the ground?

Ukrainian war crimes prosecutors are working with mobile justice teams supported by international legal experts and forensic teams. They have been investigating alleged violations of international law since shortly after the invasion started 24 February 2022 mainly in the south and east, where land has been recaptured from Russian forces.

Domestic courts are focusing on “direct perpetrator” crimes, and at least 26 war crime suspects have been tried and convicted of rape and murder, shelling of residential infrastructure, cruel treatment and pillaging, according to Ukrainian prosecutors.

But trying to hold Russian leaders and commanders accountable for actions committed on their orders will most likely take years.

“The more difficult job of trying to build complex aggregate cases which establish the responsibility of those in the higher political and military leadership is a task which still remains to be done,” said Wayne Jordash, leader of the mobile justice teams deployed to support Ukraine’s investigations.

Evidence is being amassed, however.

“What’s clear from the prosecution’s investigations over the last year is that there is a criminal plan and the Russian military operation is inherently criminal, in the sense that you cannot seek to extinguish Ukrainian identity without the massive commission of war crimes and crimes against humanity and possibly genocide,” Jordash said.

What other avenues are available

The European Union recently announced the creation of an international centre for the prosecution of “aggression” in Ukraine, which is under the European prosecuting authority Eurojust, also in The Hague. This could eventually form the basis of a new tribunal – see below.

War crimes can be defined under customary international law or national law. Ukraine’s war crime definitions are narrower than those of the ICC, for example.

A number of mostly European states have universal jurisdiction laws that would also allow them to prosecute Ukrainian war crimes.

The ICC has joined a Joint Investigative Team with Lithuania, Poland, Estonia, Latvia, Slovakia, Romania and Ukraine itself to support possible trials inside or outside Ukraine.

In addition, a United Nations Independent International Commission of Inquiry for Ukraine is collecting and documenting violations of international humanitarian law, to feed into the evidence being collected and shared at Eurojust. This could also could also support cases taken on by the ICC.

What is the crime of aggression?

The crime of aggression is broadly defined as the invasion of, or attempt to gain political and military control over, another sovereign state. While the ICC is the world’s permanent war crimes court, it cannot prosecute aggression.

“War is a crime, but not a war crime,” said Astrid Reisinger Coracini, international law lecturer at the University of Vienna and expert on the crime of aggression.

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To bridge this “impunity gap”, Ukraine and several backers, including the United States and the European Union, are pushing for an ad-hoc tribunal for aggression.

Under customary international law, heads of state, heads of government and foreign ministers are immune from prosecution before national courts, Reisinger Coracini said.

A tribunal that could prosecute Russian President Vladimir Putin for aggression would therefore have to be a new international one, founded on a source of international law through a multilateral treaty.