Beginning in late 1941, the Germans began mass transports from the ghettoes in Poland to the concentration camps, starting with those people viewed as the least useful: the sick, old and weak and the very young. The first mass gassings began at the camp of Belzec, near Lublin, on March 17, 1942. Five more mass killing centers were built at camps in occupied Poland, including Chelmno, Sobibor, Treblinka, Majdanek and the largest of all, Auschwitz-Birkenau. From 1942 to 1945, Jews were deported to the camps from all over Europe, including German-controlled territory as well as those countries allied with Germany. The heaviest deportations took place during the summer and fall of 1942, when more than 300,000 people were deported from the Warsaw ghetto alone.
As Nazi tyranny spread across Europe, the Germans and their collaborators persecuted and murdered millions of other people. Between two and three million Soviet prisoners of war were murdered or died of starvation, disease, neglect, or brutal treatment. The Germans targeted the non-Jewish Polish intelligentsia for killing, and deported millions of Polish and Soviet civilians for forced labor in Germany or in occupied Poland, where these individuals worked and often died under deplorable conditions.
Mengele worked as a carpenter in Buenos Aires, Argentina, while lodging in a boarding house in the suburb of Vicente López. After a few weeks he moved to the house of a Nazi sympathizer in the more affluent neighborhood of Florida Este. He next worked as a salesman for his family's farm equipment company, Karl Mengele & Sons, and in 1951 he began making frequent trips to Paraguay as regional sales representative. He moved into an apartment in central Buenos Aires in 1953, he used family funds to buy a part interest in a carpentry concern, and he then rented a house in the suburb of Olivos in 1954. Files released by the Argentine government in 1992 indicate that Mengele may have practiced medicine without a license while living in Buenos Aires, including performing abortions.
Executions by the Einsatzgruppen, mobile killing squads,were abandoned for practical reasons. Although approximately 1.5 million Jews had been shot by the winter of 1941, the Nazis felt that the efficiency of this slow and cumbersome method left much to be desired. Moreover, they found it was bad for the soldiers’ morale. Himmler himself, commander of the SS and as such responsible for the annihilation of the Jews, was persuaded, after having witnessed such an execution, that it badly affected the mental health of those carrying out the execution. The institutionalization of organized murder, founded on a division of labor and carried out in special installations expressly designed for this purpose, distanced the executioner from the victim, an indispensable psychological advantage in an enterprise of annihilation of such a huge scale.
Reichsführer-SS Heinrich Himmler visited Auschwitz-Birkenau on July 17 and 18, 1942 and watched the gassing of 449 women and children in Bunker No. 1, according to his biographer Peter Padfield. On July 23, 1942, Himmler ordered the quarantine of the Birkenau camp because of a typhus epidemic, but the gassing of the Jews continued. On December 28, 1942, Himmler issued an order that the death rate "must be reduced at all costs" according to document 2172-PS that was introduced at the Nuremberg IMT. He meant the death rate from typhus; the gassing of the Jews did not stop.
Before the war, Mengele had received doctorates in anthropology and medicine, and began a career as a researcher. He joined the Nazi Party in 1937 and the SS in 1938. He was assigned as a battalion medical officer at the start of World War II, then transferred to the Nazi concentration camps service in early 1943 and assigned to Auschwitz, where he saw the opportunity to conduct genetic research on human subjects. His subsequent experiments focused primarily on twins, with little regard for the health or safety of the victims.