By the mid-1950s Germany was divided into the German Federal Republic (West) and the German Democratic Republic (East). West Germany became a full member of North Atlantic Treaty Organisation (NATO) in 1955. In the same year the Soviet Union formed the Pact of Mutual Assistance and Unified Command - the Warsaw Pact. Europe was consequently divided into two military blocs. Berlin was divided between the United States, the Soviet Union, France and Britain. The city provided an easy means for dissidents to leave East Germany, which embarrassed Communist leader, Nikita Khrushchev.
In November 1958, Khrushchev, calculating NATO would not be prepared to use nuclear weapons, set an ultimatum that gave NATO powers six months to withdraw troops from the city. He planned to make a separate peace treaty with the German Democratic Republic (GDR), but in July 1959, during a conference of foreign ministers in Geneva, Khrushchev postponed his deadline.
Shortly afterwards the British Prime Minister, Harold Macmillan, visited Moscow to discuss the position with the Soviets. In a later visit to the US Khrushchev again suspended the deadline, agreeing to discuss the problem in the Paris Summit of May 1960. Shortly afterwards, however, the Soviet Union shot down an American U-2 spy plane and discussions collapsed. Western powers planned for the eventuality of another Berlin Blockade similar to that of 1948 and 1949.
The Berlin Wall was built in August 1961 and separated East Berlin from West Berlin. The East German government used it to prevent its citizens from defecting to the West.