Final answer:
The Einsatzgruppen were mobile killing squads that targeted Jewish people and others considered enemies by the Nazis, responsible for mass murders in eastern Europe during WWII.
Step-by-step explanation:
The German military unit that followed advancing armies into eastern Europe and began eliminating enemies and Jewish people was the Einsatzgruppen. These mobile killing squads, consisting of special groups of Schutzstaffel (SS) soldiers, were responsible for mass shootings and the execution of not only Jewish communities but also other groups including political enemies, the Roma, and the disabled. The Einsatzgruppen employed horrific methods such as forcing victims to dig their own graves before being shot and buried, leading to psychological problems among the soldiers. Their actions constituted the beginning stages of the Holocaust and resulted in the deaths of approximately 1 million people during the war.