How could war crimes be prosecuted in Ukraine?

The United States recently announced that Russia is committing war crimes in Ukraine. Leadership in the United Kingdom, the European Union, Australia, and Canada have also condemned Russians for attacks on civilians.

“Russian forces are indeed committing war crimes in Ukraine,” Beth Van Schaack, the ambassador-at-large for Global Criminal Justice, told reporters last week. “As with any alleged crime, it will ultimately be for a court of law to determine individual criminal responsibility for these cases.”

The International Criminal Court opened an investigation into war crimes in Ukraine earlier this month. But prosecuting individuals for war crimes is rare. Only six people have been successfully convicted and sentenced since the formation of the ICC in 2002.

What makes something a war crime? And why is it so difficult to prosecute?