In 1952, a border, similar to that between East & West Germany, had been created around the edge of the US, UK and French sectors of Berlin.
The border between the western sectors of Berlin and East Germany
However, the border between the Soviet sector of Berlin and the western sectors which ran through the streets of the city was monitored but not fully sealed.
It was therefore quite straightforward for an East German to travel to Berlin and then walk or ride the U-Bahn or S-Bahn into the western sectors.
The diagram shows that you could travel into the Soviet sector from any part of East Germany and then cross the border into West Berlin.
As the numbers crossing continued to increase, efforts were made to stop any obvious attempt to cross into West Berlin.
This image is dated 1955 and shows the standoff created by a young woman's (successful) attempt to cross the border into West Berlin
In West Berlin the East Germans were given new paperwork and then flown out of the city into West Germany using one of the western airlines.
Would be emigrants were given new papers and flown to West Germany
In November 1958 the Soviet Premier issued what was termed the 'Khruschev Ultimatum', a demand that the western allies leave Berlin within six months.
As a consequence, the status of Berlin would be that of a free, demilitarised city with the East Germans controlling all lines of communication with West Germany. This proposal was unacceptable to the US, UK and France.
The US, UK and France formed a covert planning group to prepare a response to any incursion or aggression against West Berlin.
The group's code name was LIVE OAK. It's staff prepared land and air plans to guarantee access to and from West Berlin.
With the western allies planning to face any hostile action over Berlin, the Soviet ultimatum was withdrawn in March 1959.
Positive discussions about the future of Berlin came to an end when Gary Powers's U-2 spy-plane was shot down in May 1960.
Gary Powers, his U-2 spyplane and its wreckage in a Soviet field
The flow of refugees through West Berlin continued.
It has been estimated that between 1949 and 1961 approximately 3.5 million East Germans emigrated to the West – 1/6 of East Germany's entire population - 207,000 left in the first seven months of 1961 (before the building of the Wall in August).
Most of the emigrants continued to be the young and educated. The loss was disproportionately heavy among professionals—engineers, technicians, physicians, teachers, lawyers and skilled workers.
Many farmers also fled to the west leaving large areas of land (10% of the total) uncultivated which in turn led to food shortages.
The East German economy was on the verge of collapse and drastic measures were required to stem the flow of emigrants.