The Holocaust itself began with the invasion of the Soviet Union in the summer of 1941. As German armies advanced into Soviet territory, they were followed by four teams of Einsatzgruppen - mobile killing squads - charged with killing “Jews, Gypsies, and the disabled.” The Einsatzgruppen's technique for murdering their victims consisted of marching Jews into the woods or fields and systematically shooting them. The victims would be forced to dig mass graves or ditches, to strip, and to watch as their entire community was slaughtered. Mothers would be forced to strip, then undress their children, watch their children be murdered, and then join them in the mass graves. The Einsatzgruppen and the local helpers they recruited were responsible for approximately 1 million deaths over the course of the war. The Einsatzgruppen were aided by regular Wehrmacht (German army) units and by battalions of the Order Police, a hybrid of police force and national guard mobilized for the war effort. In other words, many “regular soldiers,” not just Nazi party members, were responsible for killing innocent men, women, and children, often for days at a time and at point-blank range. This aspect of the Holocaust is today referred to as the “Holocaust by bullets,” one that was largely overlooked by historians for many decades after the war.
There were various logistical problems with this technique, however. It was hard to generalize it in urban areas already under Nazi control. Many members of the Einsatzgruppen suffered from mental breakdowns from murdering innocent people day after day. There were never very many Einsatzgruppen to begin with: four teams with about 6,000 soldiers assigned to them in total. Out of necessity, they made heavy use of auxiliary troops to do much of the actual shooting, recruited from Ukrainian, Latvian, Lithuanian, or Estonian POW camps. These auxiliaries were called “Hiwis,” an abbreviation of Hilfswilligen (“helpers”), by the Nazis. Soon, both members of the SS’s army, the Waffen SS, as well as regular soldiers of the Wehrmacht were assigned to “Jewish Actions,” the euphemism for organized massacres.
At some point between the late summer and fall of 1941, the top Nazi leadership decided to abandon earlier experiments with forced deportations and to search instead for more efficient methods of murder. Almost immediately after the implementation of the Einsatzgruppen, the head of the SS, Heinrich Himmler, ordered experiments with better means of mass murder, which resulted in Nazi technicians devising “gas vans” that killed their victims through carbon monoxide poisoning. By late fall of 1941, killing facilities were being built in the concentration camps of Majdanek and Auschwitz, both of which had been built as slave labor camps in 1940. There, the first experiments with the infamous pesticide Zyklon B were carried out on Russian POWs.