For nearly as long as there has been war, attempts have been made to govern its conduct. However, it took until the 20th century for the legal concept of a “war crime” to come into being. Thanks to an array of multilateral treaties, most notably the Geneva Conventions, military and political leaders can now be prosecuted when they commit atrocities ranging from torture to targeting civilians.

Even in antiquity, war was rarely waged completely without limits. Aristotle, for example, outlined his parameters for a “just war.” And, though fighting could be brutal among the Greek city-states, enemy soldiers customarily accorded each other a proper burial.

In ancient India, meanwhile, the so-called Law Code of Manu prohibited poison-laced and flaming weapons, as well as the killing of captives, whereas Muslim armies in 7th-century Arabia likewise made a point of sparing prisoners of war. Later on, certain European monarchs like French king Charles VII, who ruled during the time of Joan of Arc, attempted to rein in such battlefield excesses as pillaging.

Very few of these restrictions were written down. For the most part, warfare was governed by “unwritten norms and codes of behavior and notions of chivalry and that kind of thing,” says David Bosco, an associate professor of international studies at Indiana University and author of “Rough Justice_: The International Criminal Court in a World of Power Politics_.”

Change finally came in the 19th century, when countries began codifying their armies’ conduct. U.S. President Abraham Lincoln, for one, issued the so-called Lieber Code in 1863. Named for its primary author Francis Lieber, a Columbia Law School professor, the Lieber Code instructed Civil War commanders on how to treat fugitive slaves and Confederate prisoners of war. (Among other things, it also banned the murder and maiming of non-combatants, though not their subjugation through starvation.)

As Bosco points out, the Lieber Code “became quite influential internationally.” In fact, Prussia, the Netherlands, France, Switzerland and Spain were among the countries to publish similar military guidelines in its aftermath.

At the same time, a movement arose to establish international rules of combat that all countries would (hypothetically) adhere to. In 1856, the Paris Declaration Respecting Maritime Law, which banned privateering, became the first multilateral treaty to restrict wartime practices, and it was followed by the 1864 Geneva Convention, considered the basis of international humanitarian law, which covered the treatment of sick and wounded soldiers.

Four years later, the St. Petersburg Declaration barred a type of exploding bullet, while the Hague Conventions of 1899 and 1907 further defined acceptable military conduct.

At this point, the legal concept of “war crimes” still didn’t exist, nor was there any mechanism to prosecute war criminals internationally. After World War I, however, the Allies prepared to bring charges against hundreds of alleged war criminals, including the deposed German Kaiser Wilhelm II, whom they accused of committing a “supreme offence against international morality and the sanctity of treaties.”

It never panned out: Wilhelm fled to the Netherlands, which refused to extradite him, and no international war crimes trial ever took place. In 1921, a few defendants appeared before a German court, but none received more than a short sentence.

Determined not to make the same mistake after World War II, the Allies convened the first-ever international criminal tribunals in Nuremberg and Tokyo, at which dozens of German and Japanese leaders were charged with war crimes, as well as with “crimes against peace” and “crimes against humanity.” Nearly all the defendants were convicted, and 19 were sentenced to death.

“One of the key innovations of Nuremberg was to hold German leaders responsible for crimes committed against their own citizens,” not just the citizens of other countries, says Susana SáCouto, director of the War Crimes Research Office at American University’s Washington College of Law.

Additional clarity regarding war crimes came with the Geneva Conventions of 1949, which laid out specific protections for civilians, prisoners of war, shipwrecked sailors, medical personnel, and sick and wounded soldiers. Ratified by every single member state of the United Nations, “they are still the most important document for interpreting what is lawful conduct during armed conflict,” Bosco says.

Following the Nuremberg and Tokyo trials, the effort to punish war criminals internationally went into a half-century “hibernation,” Bosco says, though he points out that individual countries did file scattered cases over that time period. Then, in 1993 and 1994, the United Nations established tribunals to prosecute war crimes in the former Yugoslavia and Rwanda, respectively.

This begged the question, Bosco explains, of why separate courts were being created for each situation “rather than a permanent institution that’s simply ready to deal with these crimes when they emerge.” Multilateral negotiations ensued, and in 2002 the International Criminal Court came into being. Since then, it has brought 31 cases, all against defendants in Africa (though investigations are ongoing elsewhere in the world as well).

Most recently, in March 2022, the ICC’s chief prosecutor announced an investigation into the Russian invasion of Ukraine. (In a separate move, the U.S. Senate condemned Russian President Vladimir Putin for alleged war crimes.)

Despite some small differences of opinion among countries, Bosco says there is now broad consensus on what constitutes a war crime. Relying heavily on the 1949 Geneva Conventions, along with a 1977 update, the statute governing the ICC contains a long list of war crimes, including torture, willful killing, willfully causing great suffering, extensively destroying property, taking hostages, intentionally directing attacks against civilians, intentionally directing attacks against humanitarian aid workers, improperly using a flag of truce, pillaging, sexual violence, conscripting children, and the use of poison weapons.

SáCouto adds that to be a war crime, there must be an armed conflict—not just protests or riots.

Many of the highest-profile cases now head to the ICC, which considers itself a court of last resort. “They didn’t want to displace national courts,” Bosco says. “They wanted to keep the onus on governments to do these prosecutions themselves.”

Ideally, the existence of the ICC deters potential war criminals. Yet even staunch supporters of the court acknowledge some imperfections. Neither the United States nor China are among the 123 member states. Russia and Ukraine likewise aren’t members, though in recent years Ukraine has given the court ad hoc jurisdiction regarding its conflict with Russia.

Certain accused war criminals, such as former Sudanese President Omar al-Bashir, have not yet been handed over to the ICC. For others, such as Ratko Mladic, an army general nicknamed the “butcher of Bosnia”—who was tried before the Yugoslavia tribunal—convictions can be decades in the making. “But that’s not a good reason not to move forward,” SáCouto says. “It just might take a while.”