During World War II, the Germans' combined armed forces (Heer, Kriegsmarine and Luftwaffe) committed systematic war crimes, including massacres, mass rape, looting, the exploitation of forced labor, the murder of three million Soviet prisoners of war, and participated in the extermination of Jews. While the Nazi Party's own SS forces (in particular the SS-Totenkopfverbände, Einsatzgruppen and Waffen-SS) of Nazi Germany was the organization most responsible for the genocidal killing of the Holocaust, the regular armed forces of the Wehrmacht committed many war crimes of their own (as well as assisting the SS in theirs), particularly on the Eastern Front in the war against the Soviet Union. According to a study by Alex J. Kay and David Stahel, the majority of the Wehrmacht soldiers deployed to the Soviet Union participated in war crimes.