During World War II, the Nazi German Einsatzkommandos were a sub-group of five Einsatzgruppen mobile killing squads (term used by Holocaust historians) – up to 3,000 men total – usually composed of 500–1,000 functionaries of the SS and Gestapo, whose mission was to exterminate Jews, Polish intellectuals, Romani, homosexuals, communists and the NKVD collaborators in the captured territories often far behind the advancing German front.[1][2] After the outbreak of war with the Soviet Union with Operation Barbarossa, the Red Army began to retreat so rapidly that the large Einsatzgruppen had to be split into dozens of smaller commandos (Einsatzkommandos), responsible for systematically killing Jews and, among others, alleged Soviet partisans behind the Wehrmacht lines. After the war several Einsatzkommando officers were tried, in the Einsatzgruppen trial, convicted of war crimes and hanged.