Before the extermination camps - "Einsatzgruppen"

A question may arise: why were extermination camps created? What forced Nazi authorities to take action in order to create mass extermination centers? One of the reasons was, among others, the activity of Einsatzgruppen (units separated from SIPO and SD), i.e. special operational groups moving behind the front line.

The main task of these units was the mass extermination of Jews, communists and enemies of the system among civilians through mass shootings as well as hanging and gassing in specially constructed trucks.  Due to their size and areas of operation, the departments were appropriately divided into four operational groups: Einsatzgruppe A/B/C/ D. 

The daily mass executions of thousands of people had a very negative impact on the psyche of the torturers and were a financial burden (weapons, food, transport). Himmler also feared how those who committed crimes against defenseless civilians, often women and children, would normally function within society after the war. The most notorious crimes were committed in 1941-1942 in: Lithuania, Latvia, Estonia, Belarus, Russia and Ukraine. From 1942, the activities of the Einsatzgruppen were gradually phased out.

It is estimated that at least 900,000 people died at the hands of the Einsatzgruppen torturers, and according to some sources * the number may even reach 1,500,000 - 2,000,000 - [*] The United States Holocaust Memorial Museum. In 1948, in trial number 9 in Nuremberg, 22 group and sub-group commanders were sentenced for the criminal activities of Einsatzgruppen. 14 of them were sentenced to death. Some of the sentences were later changed into life imprisonment or many years of prison. 

I recommend the following books:

“Hitler's Death Squads: The Logic of Mass Murder”, Helmut Langerbein
“Masters of Death: The SS-Einsatzgruppen and the Invention of the Holocaust”, Richard Rhodes
“Einsatzgruppen in Poland“, Jochen Böhler, Klaus-Michael Mallmann