By the time the two men met in 1941, the mass killing of Jews was already a fact. From the moment that Hitler's legions invaded Poland in 1939, the Wehrmacht was accompanied by special SS and auxiliary Police battalions tasked with killing Jews. Before the development of chambers where Zyklon B poison gas was used, various other methods were employed, from mass shootings, burnings, batterings, hangings, burial alive and specially adapted vans that pumped carbon monoxide fumes inside to asphyxiate their passengers. These methods were eventually deemed too slow, and the killings escalated inexorably into the millions in the last years of the war with the construction of the gas chambers.