Proving war crimes is a legal long throw. Pursuit has begun in Ukraine.

Last week, soon after granting $800 million of military aid to Ukraine, President Joe Biden responded to a reporter’s question with his harshest description of Russia’s invading leader yet.

“I think he is a war criminal,” President Biden said of Russian President Vladimir Putin. 

The unexpected comment echoed a unanimous resolution the day before from the Senate, labeling Mr. Putin the same thing. In terms of accountability, though, it’s just that – a label – until proved in court. On March 23, the U.S. government issued a formal assessment that war crimes have been committed during the invasion of Ukraine. Multiple countries and international organizations are already investigating. Even if their findings resulted in charges, there’s no guarantee any Russian leaders would ever face trial.

What is a war crime? 

“The classic definition is serious violations of international humanitarian law that give rise to individual criminal responsibility,” says Rachel Kerr, a professor at King’s College London and co-director of the War Crimes Research Group. 

That means two things. One, infractions of international humanitarian law – a decades-old patchwork of treaties like the Geneva and Hague conventions – are severe. Two, a person or group of people is responsible, not a country.  

War crimes include attacks that intentionally target civilians or civilian property, or military objectives that recklessly endanger civilians. They can also include use of munitions, such as chemical weapons, that inflict severe or indiscriminate damage. 

Because war inevitably involves civilian harm, says Professor Kerr, the key rule is proportionality – any target or tool requires a proportionate military objective. Intentionally bombing a school is off-limits. But bombing a school housing enemy troops or ammunition may be more complicated.  

“It’s military necessity versus humanity,” says Professor Kerr. 

How are war crimes investigated? 

It depends on who’s investigating. Because potential war crimes are occurring on Ukrainian soil, Ukraine has unquestioned authority to investigate. But war crimes operate under universal jurisdiction – any country has the right to investigate, regardless of location. Germany, Spain, and Poland have all begun their own inquiries. 

The International Criminal Court (ICC), as well, launched an investigation on March 2, and an unprecedented 41 member states petitioned for one, says Ryan Goodman, professor of human rights law at New York University.

“I am satisfied that there is a reasonable basis to believe that both alleged war crimes and crimes against humanity have been committed in Ukraine,” wrote ICC Prosecutor Karim Khan in a statement before the investigation began. (War crimes, unlike crimes against humanity, can only occur during armed conflict, can be committed against combatants and civilians, and can be isolated incidents.)

The process is arduous, beginning with specific allegations – such as a strike on a hospital or fleeing civilians – and builds a record of what happened and who’s responsible through eyewitnesses, videos, and other verified evidence. Then investigators attempt to assess intent and information at the time of the strike, which likely requires inferences because Russia almost certainly won’t cooperate. 

“That’s not an easy task, especially with the conflict ongoing,” says Karima Bennoune, visiting professor at the University of Michigan Law School and an international human rights lawyer. “Every day there’s more things that have to be investigated, and the conflict itself is a significant obstacle to the investigation.”

Human rights groups, Ukrainian citizens, and the United Nations Human Rights Council are all helping document evidence in support. While each investigation has its own timeline, some could take years, says Professor Bennoune.

What consequences can investigations bring?

“At the moment, it’s so hard to see any,” says Professor Kerr. 

The ICC doesn’t conduct trials in absentia – defendants must be in custody. When charges are filed, that rule can amount to a travel ban because leaders won’t risk being taken into custody outside their borders. As long as they remain in power, leaders like Mr. Putin and Foreign Minister Sergei Lavrov are sheltered. 

Russia, not an ICC member, wouldn’t be the first major power to fight the court’s legitimacy. The United States isn’t a member either and has resisted investigation of Americans, says Professor Goodman. Since the war in Ukraine, the Biden administration has begun an internal review of its policy toward the ICC. 

That doesn’t mean the investigations don’t matter, Professor Bennoune says. 

“It’s very positive that there has been such a strong reaction to the reports of war crimes that we’ve seen,” she says. “I think that public outrage and response is critical to actually trying to stop further war crimes.”