Shortly after midnight on this day in 1961, East German soldiers begin laying down barbed wire and bricks as a barrier between Soviet-controlled East Berlin and the democratic western section of the city.

After World War II, defeated Germany was divided into Soviet, American, British and French zones of occupation. The city of Berlin, though technically part of the Soviet zone, was also split, with the Soviets taking the eastern part of the city. After a massive Allied airlift in June 1948 foiled a Soviet attempt to blockade West Berlin, the eastern section was drawn even more tightly into the Soviet fold. Over the next 12 years, cut off from its western counterpart and basically reduced to a Soviet satellite, East Germany saw between 2.5 million and 3 million of its citizens head to West Germany in search of better opportunities. By 1961, some 1,000 East Germans—including many skilled laborers, professionals and intellectuals—were leaving every day.

READ MORE: Why the Berlin Airlift Was the First Major Battle of the Cold War

In August, Walter Ulbricht, the Communist leader of East Germany, got the go-ahead from Soviet Premier Nikita Khrushchev to begin the sealing off of all access between East and West Berlin. Soldiers began the work over the night of August 12-13, laying more than 100 miles of barbed wire slightly inside the East Berlin border. The wire was soon replaced by a six-foot-high, 96-mile-long wall of concrete blocks, complete with guard towers, machine gun posts and searchlights. East German officers known as Volkspolizei (“Volpos”) patrolled the Berlin Wall day and night.

Many Berlin residents on that first morning found themselves suddenly cut off from friends or family members in the other half of the city. Led by their mayor, Willi Brandt, West Berliners demonstrated against the wall, as Brandt criticized Western democracies, particularly the United States, for failing to take a stand against it. President John F. Kennedy had earlier said publicly that the United States could only really help West Berliners and West Germans, and that any kind of action on behalf of East Germans would only result in failure.

The Berlin Wall was one of the most powerful and iconic symbols of the Cold War. In June 1963, Kennedy gave his famous “Ich bin ein Berliner” (“I am a Berliner”) speech in front of the Wall, celebrating the city as a symbol of freedom and democracy in its resistance to tyranny and oppression. The height of the Wall was raised to 10 feet in 1970 in an effort to stop escape attempts, which at that time came almost daily. From 1961 to 1989, a total of 5,000 East Germans escaped; many more tried and failed. High profile shootings of some would-be defectors only intensified the Western world’s hatred of the Wall.

Finally, in the late 1980s, East Germany, fueled by the decline of the Soviet Union, began to implement a number of liberal reforms. On November 9, 1989, masses of East and West Germans alike gathered at the Berlin Wall and began to climb over and dismantle it. As this symbol of Cold War repression was destroyed, East and West Germany became one nation again, signing a formal treaty of unification on October 3, 1990.

READ MORE: The Surprising Human Factors Behind the Fall of the Berlin Wall