SS-Das Reich were withdrawn to the West to refit and were converted to Panzergrenadier divisions. The SS-Panzer Corps returned to the Soviet Union in 1943 and participated in the Third Battle of Kharkov in February and March. The Holocaust The SS was built on a culture of violence, which was exhibited in its most extreme form by the mass murder of civilians and prisoners of war on the Eastern Front. Augmented by personnel from the Kripo, Orpo (Order Police), and Waffen-SS, the Einsatzgruppen reached a total strength of 3,000 men. Einsatzgruppen A, B, and C were attached to Army Groups North, Centre, and South; Einsatzgruppe D was assigned to the 11th Army. The Einsatzgruppe for Special Purposes operated in eastern Poland starting in July 1941. The historian Richard Rhodes describes them as being "outside the bounds of morality"; they were "judge, jury and executioner all in one", with the authority to kill anyone at their discretion. Following Operation Barbarossa, these Einsatzgruppen units, together with the Waffen-SS and Order Police as well as with assistance from the Wehrmacht, engaged in the mass murder of the Jewish population in occupied eastern Poland and the Soviet Union. The greatest extent of Einsatzgruppen action occurred in 1941 and 1942 in Ukraine and Russia. Before the invasion there were five million registered Jews throughout the Soviet Union, with three million of those residing in the territories occupied by the Germans; by the time the war ended, over two million of these had been murdered. The extermination activities of the Einsatzgruppen generally followed a standard procedure, with the Einsatzgruppen chief contacting the nearest Wehrmacht unit commander to inform him of the impending action; this was done so they could coordinate and control access to the execution grounds. Initially, the victims were shot, but this method proved impracticable for an operation of this scale. Also, after Himmler observed the shooting of 100 Jews at Minsk in August 1941, he grew concerned about the impact such actions were having on the mental health of his SS men. He decided that alternate methods of murder should be found, which led to the introduction of gas vans. However, these were not popular with the men, because removing the dead bodies from the van and burying them was a horrible ordeal. Prisoners or auxiliaries were often assigned to do this task so as to spare the SS men the trauma. Anti-partisan