Between 1933 and 1945, Nazi Germany and its allies established more than 40,000 camps and other incarceration sites. These camps were used for a range of purposes and included forced-labor camps, transit camps which served as temporary way stations, and killing centers built primarily or exclusively for mass murder. The Germans and their collaborators murdered more than three million Jews in the killing centers alone. Only a small fraction of those imprisoned in Nazi camps survived. Most prisoners in the early concentration camps were German Communists, Socialists, Social Democrats, Roma (Gypsies), Jehovah's Witnesses, homosexuals, and persons accused of "asocial" or socially deviant behavior. These facilities were called “concentration camps” because those imprisoned there were physically “concentrated” in one location. Following the German invasion of Poland in September 1939, the Nazis opened forced-labor camps where thousands of prisoners died from exhaustion, starvation, and exposure.
To facilitate the "Final Solution" (the genocide or mass destruction of the Jews), the Nazis established killing centers in Poland, the country with the largest Jewish population. The killing centers were designed for efficient mass murder. Chelmno, the first killing center, opened in December 1941. Jews and Roma were gassed in mobile gas vans there. In 1942, the Nazis opened the Belzec, Sobibor, and Treblinka killing centers to systematically murder the Jews of the territory in the interior of occupied Poland. The Nazis constructed gas chambers (rooms that filled with poison gas to kill those inside) to increase killing efficiency and to make the process more impersonal for the perpetrators. At the Auschwitz camp complex, the Birkenau killing center had four gas chambers. During the height of deportations to the camp, up to 6,000 Jews were gassed there each day. Jews in Nazi-occupied lands often were first deported to transit camps such as Westerbork in the Netherlands, or Drancy in France, en route to the killing centers in occupied Poland. The transit camps were usually the last stop before deportation to a killing center.