The Holocaust took place throughout Nazi German- and Axis-controlled Europe, with the most intense and systematic mass murders occurring in German-occupied Poland and other parts of Eastern Europe. It began in Germany in 1933 when Adolf Hitler came to power and spread across Europe as Nazi Germany annexed and invaded neighboring countries during World War II. The Nazis established killing centers, also known as extermination or death camps, primarily in occupied Poland, including Auschwitz-Birkenau, Chelmno, Belzec, Sobibor, and Treblinka. Jews and other victims were deported from various European countries to these camps for mass execution, primarily by poisonous gas. The Holocaust affected nearly all of Europe's Jewish population, totaling six million murdered Jews, along with millions of other victims across the continent.