On 1 May 1960 the Soviets shot down an American U2 spy plane and captured the pilot, Gary Powers.
At a summit meeting in Paris, Khrushchev demanded that the American President Dwight Eisenhower apologise - Eisenhower refused and a furious Khrushchev abandoned the summit.
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 3,000 leaving each day.