Under the direction of Reichsführer-SS Heinrich Himmler and the supervision of SS-Obergruppenführer Reinhard Heydrich, the Einsatzgruppen operated in territories occupied by the Wehrmacht (German armed forces) following the invasion of Poland in September 1939 and the invasion of the Soviet Union in June 1941. The Einsatzgruppen worked hand-in-hand with the Order Police battalions on the Eastern Front to carry out operations ranging from the murder of a few people to operations which lasted over two or more days, such as the massacre at Babi Yar with 33,771 Jews killed in two days, and the Rumbula massacre (with about 25,000 killed in two days of shooting). As ordered by Nazi leader Adolf Hitler, the Wehrmacht cooperated with the Einsatzgruppen, providing logistical support for their operations, and participated in the mass killings.