During World War II, Ferencz served in the US Army in Europe. As a member of the so-called War Crimes Branch of that army, he collected evidence for war crimes committed by Germans. That evidence was used in the trials of concentration camp personnel and Germans accused of lynching American pilots.
In 1947 he was appointed chief prosecutor of one of the subsequent Nuremberg trials, the so-called Einsatzgruppen trial. Einsatzgruppen were death squads on the Eastern Front that killed any Jew, Gypsy, or other so-called enemy of the Reich they could find. They killed over a million people. All 24 indictees in Nuremberg were convicted. Four of them were hanged.
In the 1950s, Ferencz returned to the US, where he started working as a lawyer. He also wrote books on international law and was a strong supporter of the establishment of the International Criminal Court, where all kinds of government officials can be held accountable for war crimes and crimes against humanity.
