The Nazis established an extensive system of camps - labor camps, transit camps, prisoner-of-war camps and killing centers. Following the invasion of the Soviet Union in June 1941, Hitler and the Nazis turned their policy of forced emigration, imprisonment and sporadic killing of Jews into mass murder. Central to the new policy was the systematic destruction of entire Jewish communities throughout Europe.
The murder progressed from the special killing squads, the Einsatzgruppen, who moved rapidly on the heels of the German army as it swept through cities, town and villages, to mass gassing and the burning of bodies in the killing centers. Each of the killing centers, Auschwitz-Birkenau, Majdanek, Sobibor, Belzac, Chelmno, and Treblinka was located on a main railway line in Poland. The systematic deportation, followed by the mass killing, involved the collaboration of many levels of German government and society and by most governments of German-occupied and Axis nations as well.