In late 1941, the Nazis determined that the most efficient way to annihilate the Jews of Europe would be to gas them at death camps. 

The Nazis initiated Operation Reinhard in 1941 to systematically murder Jews in the “General Government” (occupied Poland). At the Wannsee Conference in January 1942, German officials and Nazi leaders were informed of plans to gas the Jews and coordinated logistics for a Europe-wide murder operation. Throughout German-occupied Europe, Jews were rounded up by SS personnel, soldiers, police, and collaborators, then herded onto cattle cars bound for the “east” – a euphemism for the death camps – and gassed on arrival.