The Holocaust
Throughout the war, as the German army occupied (and then was gradually forced out of) most of Europe, the Nazis continued to pursue their policies of removing Jews. Their original plan, before the war, was to deport all German Jews to Madagascar, leaving only the purity of the "Aryan" race in Europe. However, with the outbreak of war, they decided instead to restrict Jews to specified ghettos.
As Germany conquered central and eastern Europe, its army began trying to separate over 4 million Jews from the rest of the civilian populations. Instead of ghettos, the Nazis and their collaborators began to arrest Jews in large groups and place them in prison camps, called concentration camps. This principle was not new to European history. Britain, for instance, had used such camps while fighting the Boer War in Africa.