According to Nazi doctrine, races were ranked as superior and inferior, with the German or Aryan race at the top and blacks and Jews at the bottom of the racial pyramid. They also believed that "the superior race" was being undermined by inferior races, such as the Jews, and that the German race must strengthen itself and eliminate its enemy in order to be reborn. For anti-Semitic Nazi leader Adolf Hitler, Jews were an inferior race, a foreign threat to racial purity and the German community. After years of Nazi rule in Germany, during which Jews were constantly persecuted, Hitler's Final Solution, now known as the Holocaust, came to fruition under the cover of World War, with the construction of mass extermination centers in concentration camps (History.com Staff,