In what year did the Holocaust start?

Hitler convinced the people of Germany that all of their problems were caused by Jews. On Kristallnacht (The Night of Broken Glass) German soldiers went through the country destroying Jewish businesses, homes and Synagogues. Jews were sent to concentration camps and killed simply because they were Jews.

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1. The first systematic killings of Jews by mobile killing units began on 25 June 1941 at Kaunas, Lithuania.
2. The first routine gassings began on 8 December 1941 at Chelmno.

Obviously, persecution and some killings had taken place earlier.