There was a lot of tension surrounding Germany's future. After the Second World War, The United States, Britain, and France had West Germany, while the Soviets controlled East Germany. Berlin, the capital, was in the East but was divided as well between east and west.
The occupying countries all agreed they wanted unification, but the Soviet Union wanted control and the others did not want to give it to them. Instead, they united Western Germany and gave it one currency, which did not make Stalin happy. This led to the Berlin Crisis, which consisted of two responses.
Berlin Blockade: The Soviet Union blocked off land and rail entrance into West Berlin in 1948. This made it impossible for people from West Berlin to get to West Germany, or supplies to get into West Berlin.
Berlin Airlift: The repsonse by the United States and Great Britain was to begin dropping in supplies.
Stalin gave up by 1949, discovering he could not stop all cooperation between the alliance.
This was the closest there was to a hot war in Europe during the Cold War. Germany stayed separated between the East and West, and the Berlin Wall was erected on August 13, 1961. Fences had existed before it, but now this permanent structure stopped people in the East from heading west.