The answer is: The Soviet Union.
After Germany´s defeat in World War II, its territory, including Berlin, its capital, was divided in four zones of occupation (France, Great Britain, USA, and the Soviet Union controlling each an area). The Eastern region fell under the control of the Soviet Union and it became part of the Eastern Bloc.
Berlin, which was fully on the side occupied by the Soviet Union, was also divided in four. For this reason, Stalin advised and helped GDR (German Democratic Republic, or Eastern Germany) build a wall in 1961 that separated and isolated West Berlin within the territory of East Germany. The wall finally came down in 1989 along with the fall of the Eastern Bloc.