President Joe Biden labeled Russias Vladimir Putin a war criminal last month amid the Kremlins ongoing invasion of Ukraine, an onslaught that has forced millions from their homes and killed more than 1,100 civilians.

The chief prosecutor of the International Criminal Court, or ICC, said shortly after the war began he would open an investigation into any possible war crimes , and the human rights chief at the United Nations said there is credible evidence that the Kremlins military action may amount to such.

But whats the international justification for labeling someone a war criminal, and how are they punished?

HuffPost spoke with Leila Sadat , the James Carr professor of international criminal law at Washington University in St. Louis, who has served as a special adviser on crimes against humanity to the ICC since 2012.

What is a war crime?

The ICC was established by an international treaty called the Rome Statute and became the first permanent international criminal court when it began sitting in 2002, with 123 member countries. War is a brutal act, but Sadat said the international community has developed a set of rules about how a war should take place to minimize harm to civilians.

The body can prosecute individuals heads of state, military members and others under three broad umbrellas of criminal acts : crimes against humanity, war crimes and genocide. War crimes range from the willful killing of civilians to torture to wanton destruction of nonmilitary targets, but the key is the intent to cause harm outside of those guidelines, known as the law of armed conflict .