April 6, 2022 4:18 PM EDT

As Russian forces have withdrawn from areas near Kyiv in recent days, photographs from those areas have shocked a world that was already aghast at the invasion of Ukraine.

Images emerged of hundreds of bodies in mass graves and in the middle of the streets—some with hands bound—in Bucha, a town the Ukrainians recently liberated outside Kyiv. Ukrainian officials believe Russian troops were shooting civilians as they were withdrawing from the town; Russia has denied that accusation. In an April 3 interview with London’s Times Radio, Ukraine’s Foreign Minister Dmytro Kuleba said he wanted the International Criminal Court to send representatives to Bucha and other liberated towns “to collect all the evidence of these war crimes” so it can be used to “prosecute those who committed these atrocities.” And on Wednesday, the U.S. and its allies announced a new wave of sanctions against Russia in response to what President Joe Biden has also called “war crimes.”

To understand what exactly constitutes a war crime, TIME talked to Michael Bryant, author of A World History of War Crimes. Here, he discusses the origins and evolution of the term war crimes and the conflicts that shaped how war crimes are prosecuted.

TIME: What, in the most general sense, are war crimes?

BRYANT: The traditional definition of war crimes are violations of the law of war. At the most basic level, war crimes are [objectionable] acts committed by combatants, either against other combatants or against noncombatants—that is, civilians—during wartime.

When does this history begin? How early did you find documentation for the prosecution of a war crime?

In English, the term arises for the first time in 1906. It was in a book called International Law by a very famous jurist named Lassa Oppenheim. The term “war crime” was first used in German in 1872 by Johann Casper Bluntschli. He thought of it as just military forces acting without orders during wartime, that was a war crime. Some say Francis Lieber, a very famous American jurist who wrote the so-called Lieber Code—which was used during the Civil War to define offenses against the law of war for Union armies—supposedly used the term “war crimes,” but he never wrote it down. People have said that he used the term orally in conversations. Lieber drafted these codes partially in response to marauding acts that were carried out by partisans in Missouri.

Read more: Building a War Crimes Case Against Putin Is Harder Than You Think

How is the way war crimes are defined in this period, in the 19th century, different from the way war crimes are defined today?

There really was no settled conception of what a war crime was. There was such a stress on the power of the nation-state to define its own conception of what’s good and what’s not good in wartime. There was a real revulsion against other states being able to intervene and tell a sovereign nation how to conduct itself during a war. At this time in the 19th century, there is no international criminal jurisprudence. Not until the 20th century—we began to see, in the community of nations, the willingness to challenge state sovereignty and insist that there are certain principles of international behavior during wartime that had to be followed, regardless of whether it was a sovereign nation or not. That idea comes later. It was kickstarted to a large extent by World War I. The severity of German war crimes during World War I was such that the Allies began to think very seriously about how to make these persons accountable.

What kind of world conflict does it take for the world to come together and lay down a definition of what a war crime is?

It was really World War II, the Nuremberg war-crimes trials. The first one, the International Military Tribunal, started in November of 1945 and ended in October 1946. And the London Charter was the foundational document of that proceeding. It sets forth all of the charges against the Nazis that will be pursued. And so that’s where I think it really starts. This is really where the modern definition of war crimes comes into being. So many of the principles of law come out of Nuremberg, such as the essence of international criminal law today, which is the notion that if you’re a head of state and you commit these sorts of crimes, you can’t just plead that you were head of state in order to avoid liability. Among the principles that are established at Nuremberg is the idea that countries are not secure from being indicted criminally under international law for what they do to their own populations.

How loaded is the term “war crime”? Has its definition depended on what certain countries see as a war crime, versus what other countries see as a war crime?

It’s really not that controversial. There’s a fairly secure anchorage for it now in international law. Where you find controversy erupting is typically over the administration of international criminal law. When people are prosecuted, like leaders of countries or military commanders, they oftentimes come from African countries or countries that are less powerful. And the criticism is that you just don’t find some other political leaders from more powerful countries and more prominent countries being prosecuted for their crimes. And I think that’s a legitimate criticism. I think it has a lot to do just with the power, the authority, of well-heeled countries, powerful countries, to shield their political and military members from prosecution by these bodies.

Read more: Russian Faces War Crime Accusations as Civilian Casualties Grow

Do all war crimes get prosecuted or is it hard to get to that point?

It’s hard to get to that point. Another traditional kind of criticism of war crimes is that—other than the U.N., and bodies created by the U.N. like the International Criminal Court, or some of these ad hoc tribunals—we just don’t have a true international police that would arrest offenders. Defining war crimes is not so much the issue anymore. It’s prosecuting them actually, administering justice, that seems to be the primary obstacle.

What is important to keep in mind about war crimes as the Russian invasion of Ukraine continues?

If you take a look at Article Five of the International Criminal Court’s 1998 statute, it talks about crimes against peace, and there’s a provision that criminalizes aggressive warfare. And the problem with prosecuting Putin or the Russians for aggressive warfare at the ICC is that they are not members of the ICC, and neither is Ukraine. And according to Article 15, one of the two parties involved in this conflict has to be a member of the ICC for the ICC to have jurisdiction over that charge. So the world community could not prosecute crimes against peace for the invasion of Ukraine, which to me is the major thing that Putin has done.

The most important thing would be documenting what’s happening. The most critical thing in any sort of conflict involving allegations of humanitarian violations is to document what’s happening. And only after we’ve assembled as much evidence as we can are we in a position then to determine what should be done. Calling for war crime trials right now, it might be a little premature. The dust hasn’t settled.

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Write to Olivia B. Waxman at olivia.waxman@time.com.

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