In 1942, as the war was slowly going against the Nazis, the Nazi Party decided to carry out the Final Solution-the mass murder of the Jews of Europe. Behind the front lines, Jews were murdered en-masse, at first in mass shootings carried out by the Einsatzgruppen and then in concentration camps such as Auschwitz and Bergen Belsen in Poland and Germany, respectively. The mass murders were carried out away from the prying eyes of the German people, since the Nazis did not want a repeat of the public backlash of the Aktion T4 program.
But what if the Holocaust started on German soil before World War II even started?
In this timeline, the Wansee Conference happens sometime in the 1930s and the decision is made to exterminate the Jews on German soil as a test run for the upcoming war. Numerous methods of extermination are carried out in parallel to the OTL process of elimination for methods of mass murder-mass execution of Jews in German forests, carbon monoxide vans become a regular sight at concentration camps like Dachau. This program of extermination moves to Austria when it is annexed in 1936 and then Czechoslovakia in 1939.
By September 1939, the majority of the Jewish population of Germany and it's territories are either dead or they have fled the country. The Nazis have refined the method of extermination and have settled upon Zyklon B as the prime method of exterminating the Jews which comes just in time as they are planning to invade Poland, the country with the densest population of Jews in all of Europe...