Following the end of World War 2, Germany was broken up and divided among the Allies as one divides war booty. The western half was occupied by the United States, the United Kingdom and France, while the eastern half went to the Soviets. Berlin itself was divided into four zones, but it was completely surrounded by the Soviet-controlled Eastern Bloc. This allowed the Soviets to act as gatekeepers controlling the flow of goods and people in and out of the capital.
Source: amusingplanet.com
