From World Book Online for Students:

At the end of World War Two, the German city of Berlin was divided into four zones of occupation with Britain, France, the United States, and the Soviet Union each controlling one zone. The Soviets installed a communist government to control their zone, East Berlin, which became part of the communist country East Germany. The western zones of Berlin remained under free, democratic rule.

By 1961, more than 1,000 East Germans were fleeing to West Berlin every day. These people chose to leave their country to escape Communist rule and seek freedom and a better living standard in the West. On Aug. 13, 1961, under orders of Soviet leader Nikita Khruszchev, East German police began building a wall of concrete and barbed wire to divide the two parts of the city. Under President John F. Kennedy the U.S. debated whether to take military action to stop the Soviets and East Germans from building the wall. Although American troops were sent to West Berlin, no action was taken and the wall was completed. This was seen as a great success for the Soviet leader, Khruszchev and an embarassment for Kennedy.

The Wall was constructed as a system of heavily fortified barriers that was about 26 miles (42 kilometers) long. It included a wall of massive concrete slabs that varied from 12 to 15 feet in height. Pipes, barbed wire, and other obstacles were installed on top of much of the wall. The East Berlin side included armed guards, guard dogs, barbed wire, electric alarms, mines, and trenches. Walls and other barriers were also built around the rest of West Berlin. The length of the barriers totaled about 110 miles (160 kilometers). In the twenty-eight years that the Berlin Wall was operated, some East Germans successfully escaped to West Berlin, but thousands were captured and approximately two hundred died trying to escape. Most of those who died were shot by border guards.

A united Berlin. In 1989, anti-communist protest movements broke out in many of the Soviet satellite nations of eastern Europe. Hungary opened up its borders with its free, democratic neighbor Austria. This led large numbers of East Germans to flee to West Germany by going on a route through Poland, Czechoslovakia, Hungary and Austria. Meanwhile, widespread protests demanding greater freedom broke out in East Germany. In November 1989—in response to the protests—the East German government ended all restrictions on travel and emigration by East Germans. As a result, the 28-year-old Berlin Wall was opened in November 1989 signalling the beginning of the end of the communist government. In October 1990, East and West Germany were united into the single non-Communist country of Germany. Berlin was reunited into a single city. By 1992, nearly all of the Berlin Wall had been removed. Several sections remain standing as memorials, but most of it was broken up for use in roadbeds and other construction projects. Parts of the wall were sold to museums and private individuals.