Auschwitz-Birkenau was the largest Nazi extermination and concentration camp, located in the Polish town of Oswiecim, 37 miles west of Cracow. One sixth of all Jews murdered by the Nazis were gassed at Auschwitz. In April 1940 SS chief Heinrich Himmler ordered the establishment of a new concentration camp in Oswiecim, a town located within the portion of Poland that was annexed to Germany at the beginning of World War II. The first Polish political prisoners arrived in Auschwitz in June 1940, and by March 1941 there were 10,900 prisoners, the majority of whom were Polish. Auschwitz soon became known as the most brutal of the Nazi concentration camps.
Heinrich Himmler's Schutzstaffel (SS) took full control of the police and the concentration camps throughout Germany in 1934–35. Himmler expanded the role of the camps to hold so-called "racially undesirable elements", such as Jews, Gypsies/Romanis/Sintis, Serbs, Poles, disabled people, and criminals. The number of people in the camps, which had fallen to 7,500, grew again to 21,000 by the start of World War II and peaked at 715,000 in January 1945.