In the years between 1949 and 1961, about 2.5 million East Germans had fled from East to West Germany, including steadily rising numbers of skilled workers, professionals, and intellectuals. Their loss threatened to destroy the economic viability of the East German state. In response, East Germany built a barrier to close off East Germans' access to West Berlin (and hence West Germany).
This barrier, the Berlin Wall, was first erected on the night of August 12–13, 1961, as the result of a decree passed on August 12 by the East German Volkskammer (“Peoples' Chamber”). The original wall, built of barbed wire and cinder blocks, was subsequently replaced by a series of concrete walls (up to 15 feet [5 metres] high) that were topped with barbed wire and guarded with watchtowers, gun emplacements, and mines. By the 1980s this system of walls, electrified fences, and fortifications extended 28 miles (45 km) through Berlin, dividing the two parts of the city, and extended a further 75 miles (120 km) around West Berlin, separating it from the rest of East Germany.
The Berlin Wall came to symbolize the Cold War's division of East from West Germany and of eastern from western Europe. About 5,000 East Germans managed to cross the Berlin Wall (by various means) and reach West Berlin safely, while another 5,000 were captured by East German authorities in the attempt and 191 more were killed during the actual crossing of the wall.
East Germany's hard-line communist leadership was forced from power in October 1989 during the wave of democratization that swept through eastern Europe. On November 9 the East German government opened the country's borders with West Germany (including West Berlin), and openings were made in the Berlin Wall through which East Germans could travel freely to the West. The wall henceforth ceased to function as a political barrier between East and West Germany.
Source: Encyclopedia Britannica