After WWII, Berlin was divided in four occupation zones (exactly the the same division pattern that had been applied to the country as a whole) and each was controlled by one of the victorious allied countries: US, UK, France and URSS. The parts of the city controlled by US, UK and France were unified and constituted West Berlin. Moreover, Berlin was embodied within the Soviet occupation zone that had become a communist republic known as the German Democratic Republic. Therefore, capitalist West Berlin was located inside a communist Republic.
The Berlin Blockade was an international crisis from the Cold War era (that confronted the Eastern bloc of communist countries leaded by the URSS and the Western bloc of capitalist nations under the influence of the US) The blockade started in 1948 and lasted almost a year. The URSS blocked all the ground means of transports that the Western allied countries could use to reach West Berlin so they organized the the Berlin airlift to carry supplies by plane. The Soviets used the blockade to bargain. Stalin stated that the blockade would be removed if the newly introduced Deutsche Mark was withdrawn from West Berlin.