At midnight on Aug. 13, 1961, East Germany sealed the borders around West Berlin.

Germany had been divided into four occupation zones in the aftermath of World War II, each administered by an allied nation: France, Britain, the U.S. and the Soviet Union.

The Soviet zone completely encircled the capital of Berlin, which was further divided among the four powers. The portion of the city controlled by France, Britain and the U.S. — West Berlin — was thus a capitalist bubble in the middle of the new Soviet state of East Germany.

East Germans fled the new regime in droves, increasingly through West Berlin, with 2,000 escaping each day by August 1961.

To stem the bleeding of economic power and educated professionals, East Germany rapidly sealed the border with barbed wire barriers, which would later be fortified into the Berlin Wall.