Cold War, this was the USSR trying to get the Western half of Berlin back, as Berlin was well within the Eastern sector of Germany, but it was agreed that it should also be divided into 4. When the Western quarters merged into West Berlin, the USSR felt threatened, hence they blockaded it in an attempt to get them out. This however failed, miserably :P
Later on they also tried the Berlin Wall to stop East Berliners migrating to the Capitalist West Berlin for a better life, but this eventually also got torn down.
