Between 1949 and 1961 an estimated 2.7 million East Germans left for West Germany, and Berlin was the centre of this process as defectors had easy access to its Western sectors
Although no agreement on the long-term fate of Berlin was reached, the ultimatum on Berlin was withdrawn by Khrushchev, and it was agreed that further negotiations would take place in Paris the following year
The USA claimed the U2 was a weather monitoring plane that had lost its way, but film retrieved by the Soviets showed Powers had been on a spying mission
Just as the 1960s started swinging, a new US President entered the White House. The Cold War was about to enter it's most critical phase, when the world would be pushed to the brink of nuclear war.
In 1959 Khruschchev's ultimatum on Berlin had been withdrawn, but Khrushchev believed he might be able to dominate the younger and inexperienced Kennedy. He reissued the ultimatum on Berlin at a conference in Vienna in June 1961 and once again gave the US six months to withdraw.
At the Yalta Conference of 1945, Germany and its capital Berlin were both divided into four zones of occupation. Berlin lay well inside the Soviet zone of occupation and was a source of tension throughout the Cold War.
On 13 August 1961, the Soviet authorities in East Germany sealed off East Berlin – their zone of occupation - by constructing a huge barbed wire barrier. This was soon replaced by a concrete wall, complete with lookout towers and armed guards who had orders to shoot anyone trying to cross into the Western sector.
The Brain Drain - thousands of East Germans had fled to the West through Berlin, leaving behind the harsh political climate and economic hardship of life under communism
Lure of the West - people living under communism in the Eastern sector could visit the West and see what capitalism offered
Espionage - Berlin was a Western island in a communist sea – an ideal place for American spies to gather intelligence on the Soviet military
Fearing the US tanks might try to break down the Wall or pass into its sector, the Soviets responded with an equal show of force and Red Army tanks pulled up to their side of Checkpoint Charlie