On 1 May 1960, the Soviets shot down an American U2 spy plane. The Americans had to admit it was a spy plane when the Soviets produced the pilot, Gary Powers.
A summit meeting in Paris had been arranged before the plane was shot down. It took place on 14 May - Khrushchev and American President Dwight Eisenhower were to discuss Berlin and the arms race.
Khrushchev demanded that Eisenhower apologise - Eisenhower refused and a furious Khrushchev abandoned the summit.
USA and USSR relations deteriorated as a result – Khrushchev looked to solve the Berlin issue in other ways.
At the Vienna Summit 1961, American President John F Kennedy refused to remove NATO troops from Berlin.
Soviet and East German leaders were determined to stop the flow of refugees to the West. It had now reached 3 million, with 3000 leaving each day.