Original: West Berliners and West Germans who had earlier fled East Germany or East Berlin, and thus could face imprisonment on entering East Germany or East Berlin, could only take flights for travel to and from West Berlin.
Simplified: As the Cold War continued, many East Germans began leaving East Germany for the West. East Germany closed the borders between East and West Germany in 1952, but did not seal off West Berlin; because there was freedom of movement between West Berlin and West Germany, Easterners could use the city as a crossing point to the West.