The haunting images of mass graves and bound corpses that emerged from a Kyiv suburb as Russian forces retreated over the weekend have triggered widespread international condemnation and prompted Western leaders to accuse Moscow of committing war crimes in Ukraine.

Ukrainian forces repelled Russian troops from Bucha, a leafy commuter town northwest of the capital, and uncovered an apparent massacre in their wake. The mayor of Bucha, Anatoly Fedoruk, said that about 270 residents had been buried in mass graves. Some were bound and executed – shot in the back of the head, he said.

It was the latest in a string of incidents that rights groups say probably violated the laws of war.

President Joe Biden said Monday that Russian President Vladimir Putin is a “war criminal” and called for more evidence-gathering to be used at a “war crime trial.”

“This guy is brutal, and what’s happening in Bucha is outrageous, and everyone’s seen it,” Biden told reporters Monday. “I think it is a war crime.”

More about Russia’s war on Ukraine

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The United States had already formally determined that members of Russia’s forces were committing war crimes in Ukraine, based on a “careful review” of public information and intelligence sources, Secretary of State Antony Blinken said in a statement last month.

In February, the chief prosecutor of the International Criminal Court at The Hague said he was opening an investigation into alleged war crimes and crimes against humanity in Ukraine.

Here’s what to know about what war crimes are and how perpetrators are prosecuted.

– What counts as a war crime?

The modern framework for assessing war crimes was born out of the Nuremberg trials following World War II. Nazi Party officials, military officers and German elites were tried on charges including crimes against humanity. At the time, the international community sought to establish guardrails that would minimize the horror of future conflicts.

People often use “war crimes” colloquially to describe a range of actions prohibited under international law during conflict, said William Schabas, a professor of international law at Middlesex University in London. But the term has a precise, technical definition, referring to violations of international law governing conduct in combat and during occupation.

Those violations are spelled out in international treaties such as the Geneva Conventions of 1949 and the Rome Statute, which established the International Criminal Court in 2002 to prosecute individuals responsible for war crimes, along with crimes against humanity and genocide – themselves complex terms with their own set of legal parameters.

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War crimes include the deliberate targeting of civilians; attacks that cause disproportionate civilian casualties given the military objective; and attacks on hospitals, schools, historic monuments and other key civilian sites. Plenty of horrific acts of violence resulting in civilian deaths would not meet the definition.

Ukrainian President Volodymyr Zelensky also alleged that Russia’s launch of a projectile that caused a fire at a nuclear power plant in southeastern Ukraine was a “war crime.” The Rome Statute specifies that intentionally launching an attack that will cause “widespread, long-term and severe damage to the natural environment” could be a crime, depending on the circumstances.

The use of certain weapons, including chemical ones, is also prohibited.

Cluster munitions, which scatter bomblets indiscriminately and leave unexploded duds that pose dangers to civilians after the conflict, are banned by many nations – but not Russia and Ukraine. Russia’s alleged use of those weapons in Ukraine, as well as “vacuum weapons,” isn’t automatically illegal, Schabas said. That determination depends on whether Russia took steps to avoid harming civilians.

There is also a separate legal category of crimes against humanity, which includes mass murder, enslavement and torture.

International law around conflict governs more than just combat. Ukraine has adopted the tactic of posting photos and videos of captured and killed Russian soldiers online, which could be a violation of the Geneva Conventions.

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– Who has the power to investigate?

Ukrainian authorities have the primary responsibility to investigate alleged violations of international law committed on Ukrainian territory, said James Gow, professor of international peace and security at King’s College London. But that would require the Ukrainian justice system being functional.

Another avenue: the International Criminal Court.

Neither Ukraine nor Russia is a party to the court, so neither can bring allegations to prosecutors. But Ukraine has twice accepted the court’s jurisdiction over its territory – and other countries can refer alleged crimes there to the court. Karim Khan, the ICC’s chief prosecutor, said in early March that his office received referrals from 39 countries.

Khan announced that he was opening an investigation into allegations of war crimes, crimes against humanity or genocide in Ukraine “by any person” from November 2013 onward.

A preliminary investigation found “reasonable basis to believe crimes within the jurisdiction of the Court had been committed” and “identified potential cases that would be admissible,” Khan said in a statement.

Separately, the United Nations’ top human rights body voted to set up a commission to probe alleged Russian rights violations during the invasion.

– What evidence is required to prosecute perpetrators?

A large quantity, and it has to be solid.

ICC investigators have begun collecting evidence, and Khan appealed for countries to give his office additional resources.

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Eliot Higgins, the founder of open-source group Bellingcat, told the Guardian that his organization was working with others to preserve evidence of potential war crimes that would be accepted in court.

The world is watching the war unfold in real time, as a flood of videos across Facebook, Telegram, TikTok and Twitter show the destruction left by apparent cluster munitions and strikes on a TV tower close to a Holocaust memorial in Kyiv. Human rights groups and journalists have verified and published eyewitness and social media accounts of attacks on civilian areas.

Social media documentation can help in investigations. But the bar for evidence in war crimes cases is high, international law experts say.

War is a bloody business, and civilian casualties are expected. In many cases, proving that the killing of civilians constitutes a war crime requires showing the attacker’s intent to harm civilians or strike forbidden targets such as hospitals and schools. So holding top officials accountable typically requires intercepting communications up the chain of command, Gow said.

“Proportionality” is a subjective standard that isn’t clearly defined, Schabas said – and Ukraine’s defense tactics, including the fortification of residential neighborhoods in Kyiv, give Russia legal cover for attacking civilian areas.

“Crimes on the battlefield are very hard to prove,” he said. “There have been very few successful prosecutions for those types of crimes.”

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Building a case is further complicated by increasingly blurred lines between civilians and fighters. Ordinary Ukrainians have taken up arms and gathered to make molotov cocktails. They lose their status as civilians under international law when they’re fighting, Schabas said.

– Could Russian officials be held accountable?

Shortly after Putin launched his invasion, calls for accountability began ricocheting across the world.

World leaders have faced trial for war crimes in the past. In a famous example, an international court put the president of the former Yugoslavia, Slobodan Milosevic, on trial in 2002 for war crimes and crimes against humanity in the Balkan wars of the 1990s. Milosevic died in custody before the trial concluded.

But even if the evidence is there, putting Russian officials – let alone Putin – on trial is unlikely, barring major political change in Russia, Gow said. As long as the Russian government remains unwilling to cooperate, and Russians accused of crimes don’t travel abroad, there’s not much international prosecutors can do.

Still, it’s valuable to collect and preserve evidence of potential war crimes and crimes against humanity, Gow said, since circumstances in Russia may change.

“No one likes to be branded a war criminal, so there is [an] important potential psychological and political effect to be gained contingently from investigations,” he said.

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And even if the perpetrators of any war crimes in Ukraine do not face punishment, stockpiling evidence can serve as a crucial corrective to false narratives.

The international tribunal that tried Milosevic said the “greatest achievement” of the proceedings was making reams of evidence available to the public as “a barrier against malign attempts to revise history.”

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The Washington Post’s Erin Cunningham, Miriam Berger and Amber Phillips contributed to this report.