Raul Hilberg, whose study remains the foundation of the field, described the Holocaust as a process that unfolded in stages: definition, expropriation, concentration, annihilation. It began not with camps but with law, the register and the identity card. From 1933 the Nazi state stripped German Jews of professions, property and citizenship; from 1938 violence became open with the November pogrom; from 1939 the conquest of Poland brought the largest Jewish population in Europe under German control and into ghettos; and from the invasion of the Soviet Union in 1941 the shooting squads of the Einsatzgruppen, and then the extermination camps built on occupied Polish soil, Auschwitz-Birkenau, Treblinka, Belzec, Sobibor, Chelmno and Majdanek, carried out the murder of some six million Jews, two thirds of the Jews of Europe.
What was destroyed was not population alone but a civilization: the Yiddish speaking world of eastern Europe, its yeshivas and Hasidic courts, its synagogues, printing houses, libraries and schools, a religious culture a thousand years in the making, murdered in four. The Roma were subjected to a parallel genocide, and disabled people, Slavic elites, Soviet prisoners, political and religious dissenters, Jehovah's Witnesses among them, and homosexual men were persecuted and killed within the same machinery.
The perpetrators were a modern European state and its willing helpers, and that fact, more than any other, is what the disciplines of history, law and theology have been answering since 1945.