For the 3.5 million East Germans who fled after World War II, the answer was obvious. Seeking to escape communist repression and reunite with their families, during the 50s and early 60s the divided city of Berlin was the standard loophole by which East Germans slipped undetected into West Germany.
That is, until 1961. Seeking to stop the continuing "brain drain"—a disproportionate number of escapees were highly educated and skilled workers—on the night of August 12, 1961, the East German government began to seal off all entry points between the two halves of the city with barbed wire.
Eventually a concrete block wall surrounded by sentry towers and minefields was erected, completely sealing off the two sections of Berlin. When the U.S. government began to make plans to bulldoze the wall, the Soviets moved armored units into position to protect it, leading to a Cold War standoff that wouldn't end until the wall's demise in 1989.
Although estimates vary, during the lifetime of the wall, between 100-200 people were killed attempting to flee East Germany. More than 5,000 escaped successfully, often resorting to risky tactics such as hiding away in luggage or flying over the wall via hot air balloon.