Like every historical event, the Holocaust evokes certain specific images. When the Holocaust is mentioned most people immediately think of the concentration camps. They immediately envision emaciated victims in dirty striped uniforms staring incomprehensibly at their liberators or piles of corpses, too numerous to bury individually, being bulldozed into mass graves.
Those are accurate images. Those horrific scenes are real. They happened. But they are not all of the Holocaust. They are merely the end product of the systematization of the genocide committed by the Third Reich. The reality of that genocide began not in the camps or in the gas chambers but with four small groups of murderers known as the Einsatzgruppen formed by Himmler and Heydrich immediately before the invasion of the Soviet Union. They operated in the territories captured by the German armies during the invasion of the Soviet Union and, with the cooperation of German army units and local militias, murdered over a million men, women, and children. It was a story that did not end until 1952 when Otto Ohlendorf, the last surviving commander of an Einsatzgruppe (Einsatzgruppe D), climbed the steps of the gallows to pay for the more than 90,000 murders his command committed.
An article at the Holocaust History Project by Yale F. Edeiken introduces the Einsatzgruppen.