At the Yalta Conference of 1945, Germany and its capital Berlin were both divided into four zones of occupation.
Berlin lay well inside the Soviet zone of occupation and was a source of tension throughout the Cold War. This had first become apparent in 1948 with the crisis over the Berlin Blockade.
The East German government, led by Walter Ulbricht, was pressurising Khrushchev to support them in weakening the power of the Western Allies in West Berlin. Khrushchev had suggested signing a peace treaty with East Germany, without the involvement of the Western Allies, which would put East Berlin under the jurisdiction of the East German government, rather than it being a Soviet zone of occupation. The East Germans would take a harder line than the Soviets on issues concerning Berlin.
On 13 August 1961, the Soviet authorities in East Germany sealed off East Berlin – their zone of occupation - by constructing a huge barbed wire barrier. This was soon replaced by a concrete wall, complete with lookout towers and armed guards who had orders to shoot anyone trying to cross into the Western sector.