The Nazi camp system expanded rapidly after September 1939, as forced labor became important in war production. Labor shortages in the German war economy became critical after German defeat in the Battle of Stalingrad. This led to the increased use of concentration camp prisoners as forced laborers in German armaments industries. Hundreds of subcamps, generally smaller camps administered by the main camps, were established in or near industrial plants and supplied them with the required workers. Camps such as Auschwitz, Buchenwald, Gross-Rosen, Natzweiler-Struthof, Ravensbrueck and Stutthof became administrative centers of huge networks of subsidiary forced-labor camps.
Source: U.S. Holocaust Memorial Museum.