https://www.nationalww2museum.org/war/articles/heinrich-himmler-holocaust
"In 1929, Hitler appointed Himmler Reichsführer of the Schutzstaffel (SS.) At the time, the SS served as bodyguards for Hitler and other top Nazi leaders. As the newly appointed leader of the SS, Himmler sought to develop an elite faction of the Nazi Party. In 1929, the SS totaled 280 men. By the time the Nazis came to power in 1933, Himmler had grown the SS to 52,000. Himmler inducted two new functions for the SSinternal security and guardianship over racial purity. In 1931, Himmler established the Sicherheitsdienst (Security Service, or SD) to gather intelligence and monitor opponents within the Nazi Party, leaders of other political parties, and government officials. In 1934, the SD was distinguished as the sole political intelligence agency for Germany. Five years later, in September 1939, Himmler fused the SD with the Reich Security Main Office (RSHA.)
Following the Nazis invasion of Poland, in 1939 Hitler appointed Himmler as the Reich Commissar for the Strengthening of the German Ethnic Stock. This new position authorized Himmler and the SS to have complete control over German resettlement in areas of occupied Poland and eventually the Soviet Union in 1941. Himmler had complete say over who was German, where ethnic Germans should live, and which populations should be moved out or destroyed to make room for resettled Germans. Himmlers plan for resettlement would be conducted with the help of the Einsatzgruppen, also known as the SSs mobile killing units. The Einsatzgruppen was tasked to initiate and partake in mass murder of Jews, government officials, Roma, and people with disabilities, no matter the age or gender.
Himmlers atrocities did not stop in the Soviet Union. Within the history of World War II, Himmler is best known for his role in the implementation of the Nazi concentration camp system and the Holocaust. In 1933, Himmler established the Nazis first concentration camp in an abandoned munitions factory at Dachau. Initially, concentration camps were made for the purpose of housing political prisoners. The first prisoners of Dachau consisted of communists, social democrats, and other political opponents to the Nazi regime. Hitler, impressed with Dachau, authorized Himmler to establish an entire system of concentration camps. By the end of the war, the Nazi camp system grew to contain thousands of camps including concentration camps, forced labor camps, POW camps, transit camps, and killing centers."