Germany's capital, Berlin, was deep within the area controlled by the Soviet Union after World War II. Initially governed in four sectors controlled by the four Allied powers (United States, United Kingdom, France and the Soviet Union), tensions of the Cold War escalated until the Soviet forces implemented the Berlin Blockade of the city's western sectors, which the Western allies relieved with the dramatic airlift. Afterward, the sectors controlled by the NATO Allies became an effective exclave of West Germany, completely surrounded by East Germany. Starting in 1952, the border between East and West was closed everywhere but in Berlin. Hundreds of thousands of East Germans defected to the West via West Berlin, a labour drain that threatened East Germany with economic collapse.