The Allies (NATO) did not refuse to leave Berlin. In fact, the Allies were occupying West Berlin in order to protect it from Soviet aggression during the Cold War. The Soviet Union had blockaded West Berlin in an attempt to force the Allies to abandon the city, but the Allies responded with the Berlin Airlift, which provided food and supplies to the people of West Berlin by air. The Allies eventually won the standoff, and the Soviet Union lifted the blockade in 1949. I believe this answers your question.