During the Second World War Nazi-Germany was responsible for the killing of approximately six million Jews. Although the gas chambers of Auschwitz-Birkenau, Sobibor and other extermination camps played a significant role in the extermination program, the mass murder had already started before these killing centers even existed. During Operation Barbarossa, the German invasion of the Soviet-Union in June 1941, the troops of the Wehrmacht were followed by four so-called Einsatzgruppen der Sicherheitspolizei und des Sicherheidsdienstes (Einsatzgruppen or Special Action Squads), special armed units of the SS which were officially charged with police and safety matters in the military occupation zones.
These fast moving teams carried out arrests and mass executions anywhere they appeared; apart from Jews, Communists, gypsies and psychiatric patients fell victim to them as well. During the Nazi occupation of the Soviet-Union, in addition to the victims of the mass extermination in the camps, more than 1 million Soviet Jews were killed, mainly by shooting. Many of these executions were carried out by the Einsatzgruppen, assisted by local collaborators.