By the 1960s Berlin was still divided - the USSR controlled the East and the USA guaranteed freedom in the West. Thousands of refugees escaped to West Berlin each day - much to the embarrassment of the USSR - so in 1961 Khrushchev closed the border and ordered the construction of a wall to stop people leaving.
The problems in West Berlin
West Berlin was a worry and an embarrassment for the Soviet Union in 1961:
Nearly 2,000 refugees a day were fleeing to the West through west Berlin - hardly proof of the Soviet claim that the Communist way of life was better than capitalism!
The Soviets believed (rightly) that West Berlin was a centre for US espionage.
At the Vienna Summit of June 1961, therefore, Khrushchev demanded that the US leave West Berlin within six months. Kennedy refused and instead guaranteed West Berlin's freedom.
On 13 August, Khrushchev closed the border between East and West Berlin and started building the Berlin Wall. At first, the Russians regarded it as a propaganda success, but as time went on, it became a propaganda disaster - a symbol of all that was bad about Soviet rule.