the start of the holocaust
The holocaust started very slowly. Years of difficult to change theological anti-Judaism led to cultural anti-Semitism which resulted in the desire to eliminate the Jews from within Christian lands. The extermination aspect of the Holocaust started in earnest in 1941, after the German invasion of the Soviet Union, when the Germans started the systematic process of murdering all Jews under their authority first by shooting them and then by gassing them.
After the invasion of Poland in 1939 the Germans established ghettos in several Polish cities, where Jews were well imprisoned. Living conditions in the cramped ghettos was appalling, and disease, hunger and overcrowding killed tens of thousands of people. This is how the Holocaust started.