When Germany invaded Poland in September 1939, triggering World War II, Hitler ordered that the Polish leadership and intelligentsia be destroyed. The area around Auschwitz was annexed to the German Reich, as part of first Gau Silesia and from 1941 Gau Upper Silesia. The camp at Auschwitz was established in April 1940, at first as a quarantine camp for Polish political prisoners. On 22 June 1941, in an attempt to obtain new territory, Hitler invaded the Soviet Union. The first gassing at Auschwitz—of a group of Soviet prisoners of war—took place around August 1941. By the end of that year, during what most historians regard as the first phase of the Holocaust, 500,000–800,000 Soviet Jews had been killed in mass shootings by a combination of German "Einsatzgruppen", ordinary German soldiers, and local collaborators. At the Wannsee Conference in Berlin on 20 January 1942, Reinhard Heydrich outlined the Final Solution to the Jewish Question to senior Nazis, and from early 1942 freight trains delivered Jews from all over occupied Europe to German extermination camps in Poland: Auschwitz, Bełżec, Chełmno, Majdanek, Sobibór, and Treblinka. Most prisoners were gassed on arrival.