Experiments took place at Auschwitz, Buchenwald, Dachau, Natzweiler-Struthof, Neuengamme, Ravensbrück, Sachsenhausen, and elsewhere.
Some dealt with sterilization of men and women, the treatment of war wounds, ways to counteract chemical weapons, research into new vaccines and drugs, and the survival of harsh conditions. One of Mengele's assistants said in 1946 that he was told to send organs of interest to the directors of the "Anthropological Institute in Berlin-Dahlem".
Paramilitary death squads called Einsatzgruppen, in cooperation with the German Army and local collaborators, murdered around 1.3 million Jews in mass shootings and pogroms between 19.
By mid-1942, victims were being deported from ghettos across Europe in sealed freight trains to extermination camps where, if they survived the journey, they were worked to death or gassed.
Eventually thousands of camps and other detention sites were established across German-occupied Europe.
The segregation of Jews in ghettos culminated in the policy of extermination the Nazis called the "Final Solution to the Jewish Question", discussed by senior Nazi officials at the Wannsee Conference in Berlin in January 1942.
Discussions at the Wannsee Conference in January 1942 made it clear that the German "final solution of the Jewish question" was intended eventually to include Britain and all the neutral states in Europe, including Ireland, Switzerland, Turkey, Sweden, Portugal, and Spain.
Historians increasingly view the Holocaust as a pan-European phenomenon, or a series of holocausts impossible to conduct without the help of local collaborators.
The persecution and murder of other groups was much less consistent.
For example, he writes, the Nazis regarded the Slavs as "sub-human", but their treatment consisted of "enslavement and gradual attrition", while "some Slavs—Slovaks, Croats, Bulgarians, some Ukrainians—[were] allotted a favored place in Hitler's New Order".