The plans to exterminate all the Jews of Europe was formalized at the Wannsee Conference, held at an SS guesthouse near Berlin, on 20 January 1942. The conference was chaired by Heydrich and attended by 15 senior officials of the Nazi Party and the German government. Most of those attending were representatives of the Interior Ministry, the Foreign Ministry, and the Justice Ministry, including Ministers for the Eastern Territories. At the conference, Heydrich indicated that approximately 11,000,000 Jews in Europe would fall under the provisions of the "Final Solution". This figure included not only Jews residing in Axis-controlled Europe, but also the Jewish populations of the United Kingdom and of neutral nations (Switzerland, Ireland, Sweden, Spain, Portugal, and European Turkey). Eichmann's biographer David Cesarani wrote that Heydrich's main purpose in convening the conference was to assert his authority over the various agencies dealing with Jewish issues. "The simplest, most decisive way that Heydrich could ensure the smooth flow of deportations" to death camps, according to Cesarani, "was by asserting his total control over the fate of the Jews in the Reich and the east" under the single authority of the RSHA. A copy of the minutes of this meeting was found by the Allies in March 1947; it was too late to serve as evidence during the first Nuremberg Trial, but was used by prosecutor General Telford Taylor in the subsequent Nuremberg Trials.
Several scholars have suggested that the Final Solution began in the newly formed district of Bezirk Bialystok. The German army took over Białystok within days. On Friday, 27 June 1941, the Reserve Police Battalion 309 arrived in the city and set the Great Synagogue on fire with hundreds of Jewish men locked inside. The burning of the synagogue was followed by a frenzy of killings both inside the homes around the Jewish neighbourhood of Chanajki, and in the city park, lasting until night time. The next day, some 30 wagons of dead bodies were taken to mass graves. As noted by Browning, the killings were led by a commander "who correctly intuited and anticipated the wishes of his Führer" without direct orders. For reasons unknown, the number of victims in the official report by Major Weis was cut in half. The next mass shooting of Polish Jews within the newly formed Reichskommissariat Ostland took place in two days of 5–7 August in occupied Pińsk, where over 12,000 Jews died at the hands of Waffen SS, not the Einsatzgruppen. An additional 17,000 Jews perished there in a ghetto uprising crushed a year later with the aid of Belarusian Auxiliary Police.
Mengele was born on 16 March 1911 to Walburga (née Hupfauer) and Karl Mengele in Günzburg, Bavaria, Germany. He was the oldest of three children; his two younger brothers were Karl Jr. and Alois. Their father was founder of the Karl Mengele & Sons company, producers of farm machinery. Josef was successful at school and developed an interest in music, art, and skiing. He completed high school in April 1930 and went on to study philosophy in Munich, where the headquarters of the Nazi Party were located. In 1931, Mengele joined the Stahlhelm, Bund der Frontsoldaten, a paramilitary organization that was absorbed into the Nazi Sturmabteilung (Storm Detachment; SA) in 1934.