The barrier was built in response to the flight of about 2.5 million East Germans to West Germany in the years 1949–61. First erected on the night of Aug. 12–13, 1961, it developed into a system of concrete walls topped with barbed wire and guarded with watchtowers, gun emplacements, and mines.Following the dissolution of the Soviet Union and diminishmentof the East Block, the Berlin Wall, which divided Berlin into two halves after World War II, was demolished by thousands of East Berliners on September 12, 1990, paving the ground for the reunification of Germany. On September 12, 1990 the four victors of World War II agreed to Germany’s reunification and after 45 years this country returned to its prewar borders.