SS-"Obergruppenführer" Greiser actively participated in the Holocaust. Early in 1940, Greiser is on record challenging Hermann Göring over efforts to delay the expulsion of Łódź Jews to Poland. On 18 September 1941, "Reichsführer-SS" Himmler informed Greiser that he intended to transfer 60,000 Czech and German Jews to the Łódź ghetto until spring 1942, when they would be "resettled". The first transport arrived a few weeks later, and Greiser sought and received permission from Himmler to kill 100,000 Jews in his area. He then instructed HSSPF Wilhelm Koppe to manage the overcrowding. Koppe and SS-"Sturmbannführer" Herbert Lange proceeded to manage the problem by experimenting at a country estate at Chełmno nad Nerem with gas vans, establishing the first extermination unit which ultimately carried out the mass murder of approximately 150,000 Jews between late 1941 and April 1942. Furthermore, on 6 October 1943 Greiser hosted a national assembly of senior SS officers in Posen at which Himmler candidly spoke of the mass executions of civilians (the infamous Posen Speech). Greiser's mass murder operations were coordinated by SS-"Oberführer" Herbert Mehlhorn.