Over the weekend, as Ukrainian forces retook the Kyiv region and Russian troops began retreating to (and expanding fighting in) eastern and southern Ukraine, gruesome images emerged of dead civilians littering the streets of Bucha, a suburb of Kyiv.

According to Ukrainian officials and independent reports, the victims included not just fighting-age men, but countless women, children, and elderly people. Hundreds had been allegedly beaten, raped, tortured, and tied up by Russian soldiers before being executed and left to rot on the street, buried in mass graves, or burned. Others were shot in the back and killed while riding their bikes and carrying groceries, for no apparent military reason.

The bodies of more than 400 civilians have been recovered from towns surrounding Kyiv that were recently under Russian occupation so far, according to Ukraine’s prosecutor-general Iryna Venediktova.

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Russia has denied responsibility for the killings, claiming (without evidence) that they occurred after its soldiers withdrew from the area and were staged by Ukrainian forces as a “provocation.” Russia’s Defense Ministry said that "not a single civilian has faced any violent action by the Russian military" in Bucha. However, satellite images and eyewitness and survivor accounts suggest that the atrocities were in fact committed by occupying Russian troops in the weeks prior to their retreat from the region.Ukrainian President Volodymyr Zelensky, who visited Bucha on Monday, called the Russian actions there a “genocide” against the Ukrainian people and nation. "We are citizens of Ukraine, and we don’t want to be subdued to the policy of [Russia]," he said in an interview with CBS’ Face the Nation on Sunday. "This is the reason we are being destroyed and exterminated."

Not everyone in the international community shares Zelensky’s assessment.

President Biden condemned the killings and called for an international investigation and a trial of Russian President Vladimir Putin, whom he had called a “war criminal” weeks before reports of the Bucha massacre emerged. "[Putin] is brutal and what's happening in Bucha is outrageous and everyone's seen it," Biden said on Monday.

However, while he and his national security advisor Jake Sullivan described the murders as “war crimes,” they refused to classify them as genocide, saying the US had “not yet seen a level of systematic deprivation of life of the Ukrainian people” that would warrant the designation.So did NATO Secretary General Jens Stoltenberg. "It is a brutality against civilians we haven't seen in Europe for decades, and it's horrific and it's absolutely unacceptable that civilians are targeted and killed," Stoltenberg told CNN’s State of the Union on Sunday.

Most other Western leaders have also denounced the killings as war crimes, with German Foreign Minister Annalena Baerbock calling for those responsible to be held accountable, UK Foreign Secretary Liz Truss demanding an International Criminal Court (ICC) investigation, and French President Emmanuel Macron tweeting that “the Russian authorities will have to answer for these crimes.”

International law considers an army to have committed “war crimes” if it is found to have intentionally and knowingly killed, mutilated, raped, tortured, deported, imprisoned, or otherwise mistreated civilians and certain types of combatants (e.g., captured, surrendered, wounded) in the course of war.

“Crimes against humanity” are defined as comprising similar acts (e.g., murder, torture, rape, etc.) but only “when committed as part of a widespread or systematic attack directed against any civilian population, with knowledge of the attack” and “in furtherance of a State or organizational policy to commit such attack.” That’s a higher bar.

Finally, “genocide” encompasses a more limited set of acts committed “with intent to destroy, in whole or in part, a national, ethnical, racial or religious group,” including: killing members of the group, causing serious bodily or mental harm to members of the group, deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part, imposing measures intended to prevent births within the group, and forcibly transferring children of the group to another group.

Both war crimes and crimes against humanity refer to the targeting of individuals. The main difference between these two types of crime is one of scale and organizational premeditation. By contrast, genocide entails an effort to destroy a particular group of people. It is considered the gravest crime under international law.

What happened in Bucha?

There’s little doubt that Ukrainian civilians were unlawfully murdered by Russian forces in Bucha. At a minimum, these actions constitute war crimes, since it’s evident that many of those killed weren’t combatants and were not killed as part of a military operation.The key open questions are how widespread and systematic the Russian crimes are, how high up the responsibility for them goes, and whether the victims were targeted as individuals who were in the wrong place at the wrong time or as members of a national group (Ukrainians) that the perpetrators intended to destroy by virtue of belonging to such group.

The first question is likely to be answered in the coming weeks, as more parts of Ukraine are (hopefully) liberated from Russian control and further evidence of atrocities comes to light. Already we’ve seen a pattern of horrific abuses committed by Russian troops against civilians in many of the towns they occupied aside from Bucha.Regarding the question of responsibility, given the way the Russian state works, it is plausible that President Putin himself ordered or acquiesced to the commission of the crimes. Proving that is another matter entirely.

As for the third question, there’s not enough evidence to say one way or the other (yet). Could the facts on the ground we’ve seen so far be consistent with genocide? Sure. But they are also consistent with war crimes and crimes against humanity, and in no way can physical evidence be probative of genocidal intent.What might, however, be suggestive of it are certain Russian state-sanctioned statements that speak to the motivations behind the campaign in Ukraine. Such is the case of an article titled “What Russia must do with Ukraine” published in the Russian state-owned outlet RIA Novosti on April 3, which lays out a case for why the “de-nazification” of Ukraine—one of Russia’s primary objectives in the war—necessarily requires “de-Ukrainianization”. In other words, the elimination of the Ukrainian nation. If this document reflects the Kremlin’s thinking, then it’d be evidence, albeit not conclusive evidence, of genocidal intent. But short of that, judgement should be withheld. Intent can be incredibly difficult to figure out even with the benefit of hindsight, and we are still very much in the fog of war. This is a question for tomorrow’s historians.

The International Criminal Court, established by the Rome Statute in 1998, is the body with jurisdiction over individuals accused of war crimes, crimes against humanity, and genocide. Neither Russia, Ukraine, nor the US are state parties to the ICC (which makes the legitimacy of US government claims of war crimes more challenging), but Ukraine did accept the jurisdiction of the court over crimes committed on its territory following Russia’s annexation of Crimea and invasion of the Donbas in 2014. This means that technically, the ICC has the authority to go after any atrocities Russia may have committed in Ukraine since then.

In fact, in late February (before Bucha), the ICC opened an investigation into alleged war crimes and crimes against humanity in Ukraine. And on March 4, the UN Human Rights Council established an independent inquiry commission to investigate alleged Russian human rights violations in Ukraine.

Yet war crimes, crimes against humanity, and genocide have all been historically difficult charges to investigate and prosecute. It’s hard to prove beyond a reasonable doubt that a given act intentionally and knowingly targeted civilians, and it’s even more challenging to determine who along the chain of command was responsible or complicit. Tying high-ranking officials like President Putin to the crimes would require strong evidence, such as direct orders or eyewitness testimony, linking the officials to the crimes in question.

Even if Putin and/or other high-level officials could be shown to have ordered or been aware of the crimes, the ICC has no jurisdiction to arrest people in Russian territory. Since international criminal trials cannot occur in absentia, Putin or other Russian officials would have to be arrested in a country that accepts the jurisdiction of the court before they could be brought to trial.

It's accordingly unlikely that Putin ever does jail time for his crimes in Ukraine as long as he remains in power. That’s not to say that he can’t be deposed and extradited by a future, more human rights-minded Russian government. Remember, there’s no statute of limitations for war crimes.

Practically, not much, for the reasons explained above.

But symbolically, casting a world leader as a war criminal carries meaningful policy consequences. For one, it projects moral clarity and sends a message to the victims and perpetrators that what happened is beyond the pale of acceptable international norms and standards. It ensures that Putin’s break with the West is complete and irreversible. And it pressures the international community into acting more aggressively to bring about an end to the war, at the same time as it lowers the odds of a negotiated settlement and sanctions relief for Russia.

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