West Berlin was a temptation to East Berliners. In the west the Marshall Plan was beginning to make life much better. Already East Berliners and East Germans were trying to escape to the west.
By the summer of 1961, 10,000 people were escaping to the West every week.
Khrushchev refused to discuss unification and in April 1960 threatened another blockade.
Khrushchev believed that Kennedy was young and inexperienced after they met in Vienna in June 1961.
West Berlin was very awkward for the Soviet Union and East Germany. It allowed people behind the Iron Curtain an opportunity to see what life was like in the West.
In August 1961, Khrushchev believed that he could get away with building the Wall because John F Kennedy, the US president had been made to look foolish over the Bay of Pigs invasion.
This was the first time that any restriction had been placed on travel between the four sectors in Berlin.
It had always been possible for Berliners to travel from one part of the city to another. Many worked in one sector and lived in another. It was easy for them to see what life was like on the other side.
Consequently hundreds of thousands of people had escaped from East to West, since the end of the Blockade in May 1949. On average the number ranged from 20-25,000 each month.
In November 1958 Khrushchev demanded that the three western powers should leave West Berlin. The West refused and called for talks on the reunification of East and West Germany (in fact at the Rome Olympics in 1960 there was a united German team, but nothing more).
They were just the sort of people that the Communist Bloc could not afford to lose as it tried to modernise its industry and agriculture.
In September 1960, East Germany forced West Berliners who wanted to travel to East Berlin to obtain a police pass.