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Shocking atrocities in Ukraine, allegedly at the hands of Russian forces, have amplified calls to pursue war crimes charges against Russian President Vladimir Putin.

Images of at least 20 bodies strewn across the street in Bucha, Ukraine, emerged in early April following the withdrawal of Russian forces from the area, prompting Ukrainian President Volodymyr Zelensky to call for an end to Russian “war crimes.” And at least 50 deaths from a Russian missile strike on a railway station in the eastern city of Kramatorsk – where civilians were waiting for evacuation trains to safer regions of the country – led Zelensky to say, “We expect a firm, global response to this war crime.”

Already, there was a loud and growing chorus of calls for the International Criminal Court to pursue Putin for the unprovoked invasion, and the US government in March formally declared that members of the Russian armed forces have committed war crimes.

The top war crimes prosecutor for the ICC has traveled to Ukraine to investigate, and the US Embassy in Kyiv argued in the war’s opening days that specific Russian attacks constituted war crimes.

“It is a war crime to attack a nuclear power plant,” the embassy said on its official Twitter feed on March 4. “Putin’s shelling of Europe’s largest nuclear plant takes his reign of terror one step further.”

Russia’s bombing of hospitals and a theater where children were seeking refuge along with its suspected use of cluster bombs and so-called vacuum bombs in dense areas with many civilians have also been described as war crimes.

“The law is clear on this, it is a crime to intentionally target civilians, it is a crime to intentionally target civilian objects,” Karim Khan, the ICC’s chief prosecutor, told CNN’s Anderson Cooper.

But Khan added there is a burden of proof and a process that must play out.

Here’s a very broad look at war crimes and the international justice movement.

Note: Some of what’s below comes from CNN’s research library, which compiled information about the International Criminal Court.

What is a war crime?

The International Criminal Court has specific definitions for genocide, war crimes, crimes against humanity and the crime of aggression. Read about them in this guide published by the ICC.

Specifically, targeting civilian populations, violating the Geneva Conventions, targeting specific groups of people and more could be potential Russian war crimes.

Khan said there can be justified attacks in civilian areas if they are being used to launch attacks. But even then, he said, attacks in civilian areas cannot be disproportionate.

There is a method of gathering evidence from testimony, satellite images and elsewhere to meet a burden of proof.

What is the International Criminal Court?

Located in The Hague, Netherlands, and created by a treaty called the Rome Statute first brought before the United Nations, the International Criminal Court operates independently.

Most countries on Earth – 123 of them – are parties to the treaty, but there are very large and notable exceptions, including Russia and the US. And, for that matter, Ukraine.

Who can be tried by the court?

Anyone accused of a crime in the jurisdiction of the court, which includes countries that are members of the ICC, can be tried. The court tries people, not countries, and focuses on those who hold the most responsibility: leaders and officials. While Ukraine is not a member of the court, it has previously accepted its jurisdiction.

Putin could, therefore, theoretically be indicted by the court for previously ordering war crimes in Crimea.

However, the ICC does not conduct trials in absentia, so he would either have to be handed over by Russia or arrested outside of Russia. That seems unlikely.

What crimes does the court handle?

The ICC is meant to be a court of “last resort” and is not meant to replace a country’s justice system. The court, which has 18 judges serving nine-year terms, tries four types of crimes: genocide, crimes against humanity, crimes of aggression and war crimes.

How does the ICC bring proceedings?

Court proceedings can be brought in one of two ways: Either a national government or the UN Security Council can refer cases for investigation.

Russia, a permanent member of the UN Security Council, has veto power over council actions. It was requests by 39 national governments, most of them European, that sparked this current investigation.

Khan previously told CNN, “I want to emphasize that I’m willing to speak to all sides, and not just the Ukrainian side, but also the Russian Federation, state parties and non-state parties alike. This institution is not political. We’re not part of the geostrategic or geopolitical divisions that we witness around the world.”

What will the ICC investigate in relation to Ukraine?

In its new investigation into Russia’s possible war crimes, the ICC has said it will look at all actions in Ukraine from 2013 to the present.

Russia first entered Crimea, which has been part of Ukraine, in 2014. The ICC was already investigating crackdowns on protesters by a previous Ukrainian government that was pro-Russian. This new referral seems to put all potential war crimes together.

How long do these investigations take?

If justice in general moves slowly, international justice barely moves at all. Investigations at the ICC take many years. Only a handful of convictions have ever been won.

A preliminary investigation into the hostilities in eastern Ukraine lasted more than six years – from April 2014 until December 2020. At the time, the prosecutor said there was evidence of war crimes and crimes against humanity. Next steps were slowed by the Covid-19 pandemic and a lack of resources at the court, which is conducting multiple investigations.

That perception of slow and ineffective justice will test the system of international law, Khan told Cooper.

“This is a test for the court. It’s a test for me, it’s a test for the office,” he said.

What are cluster bombs and vacuum bombs?

In addition to attacks on hospitals and civilian apartment buildings, the feared use of banned weapons meant to kill without discrimination is another very specific war crime.

With a cluster bomb, a missile is fired and explodes thousands of feet in the air, releasing smaller bombs that each detonate when they fall to the ground. See an illustration from The Washington Post. Amnesty International said a Russian cluster bomb fell on a Ukrainian preschool.

“Vacuum bombs,” or thermobaric weapons, suck in the oxygen from the surrounding air to generate a powerful explosion and a large pressure wave that can have enormous destructive effects. Russia previously used them in Chechnya.

‘This is genocide’

Now, Russia is facing more accusations of war crimes after its forces began retreating from areas near Kyiv, including Bucha, following a failed bid to encircle the capital.

Asked during an appearance on CBS in early April if Russia was carrying out genocide in his country based on the images out of Bucha, Zelensky said: “Indeed. This is genocide.”

The alleged atrocities in Bucha have drawn outrage from leaders outside of Ukraine, with Western leaders, including US Secretary of State Antony Blinken, calling for war crimes investigations and increasing sanctions on Russia.

“Since the aggression, we’ve come out and said that we believe that Russian forces have committed war crimes, and we’ve been working to document that, to provide the information we have to the relevant institutions and organizations that will put all of this together. And there needs to be accountability for it,” he told CNN.

CNN has not been able to independently confirm the details around the deaths.

Why would a Ukraine prosecution be different?

The international outcry against Russia is unique, and that could give the court the ability to operate differently, according to Ryan Goodman, a law professor at New York University and co-editor-in-chief of Just Security, an online forum.

“It’s hard to judge the ICC’s investigation based on past practice,” Goodman said in an email after the court initially launched its investigation. “In the Ukraine situation, the prosecutor is buttressed by an extraordinary outpouring of support from dozens of countries, which I expect will be followed by an infusion of resources.”

How would an ICC case affect the conflict?

“For better or for worse, the ICC investigation may affect the diplomatic space for negotiations,” said Goodman, arguing Putin and other Russians might not want to risk arrest if they travel outside the country.

The investigation could also, he argued, weaken Putin at home.

“Russians may come to realize this is another reason Putin can no longer serve their country,” Goodman said.

What happened before the ICC?

Previous trials for war crimes were brought by special UN tribunals, such as those empaneled for the former Yugoslavia, focusing on the Serbian autocrat Slobodan Milosevic, and for the Rwandan genocide.

All of this stems from the precedent of the Nuremberg trials to bring Nazis to justice after World War II and held by the Allies, including the US, the Soviet Union, France and Germany.

So it is interesting that neither the US nor Russia are members of the ICC.

Why aren’t the US and Russia ICC members?

Both the US and Russia are signatories to the treaty that created the court – meaning their leaders signed it – but neither is a member of the court.

Russia pulled out of the court in 2016 days after an ICC report published what CNN called a “damning verdict” on Russia’s occupation of Crimea in 2014. The court also launched a probe in 2016 into Russia’s 2008 efforts to support breakaway regions in Georgia.

At the time, France had also accused Russia of committing war crimes in Syria.

As for the US, while President Bill Clinton signed the treaty creating the court in 2000, he never recommended the Senate ratify it.

The George W. Bush administration, to a fair amount of criticism, pulled the US from being a party to the treaty in 2002. The Pentagon and many US policymakers have long opposed joining such an international court system since it could open US servicemembers to allegations of war crimes.

“The president (George W. Bush) thinks the ICC is fundamentally flawed because it puts American servicemen and women at fundamental risk of being tried by an entity that is beyond America’s reach, beyond America’s laws and can subject American civilians and military to arbitrary standards of justice,” then-White House press secretary Ari Fleischer said at the time.

How has the US supported the court?

Opposing America joining the court did not mean the Bush administration opposed the court itself. It supported ICC efforts to seek justice for genocide in Sudan.

There has always been an awkwardness to how American presidents deal with the court, noted CNN’s Tim Lister in 2011. He wrote about Barack Obama applauding ICC efforts to bring justice to people like former Serb Gen. Ratko Mladic and Libyan leader Moammar Gadhafi, while not endorsing the court for oversight of the US.

This story has been updated with additional developments.