Map of West and East Berlin, border crossings, metro networks (interactive map)
The Potsdam Agreement established the legal framework for the occupation of Germany in the wake of World War II. According to this agreement, Germany would be formally under the administration of four Allies (the United States, the United Kingdom, the Soviet Union, and France) until a German government "acceptable to all parties" could be established. The territory of Germany, as it existed in 1937, would be reduced by most of Eastern Germany thus creating the former eastern territories of Germany. The remaining territory would be divided into four zones, each administered by one of the four allied countries. Berlin, which was surrounded by the Soviet zone of occupation—newly established in most of Middle Germany—would be similarly divided, with the Western Allies occupying an enclave consisting of the western parts of the city. According to the agreement, the occupation of Berlin could end only as a result of a quadripartite agreement. The Western Allies were guaranteed three air corridors to their sectors of Berlin, and the Soviets also informally allowed road and rail access between West Berlin and the western parts of Germany (see section on traffic).
At first, this arrangement was intended to be of a temporary administrative nature, with all parties declaring that Germany and Berlin would soon be reunited. However, as the relations between the Western Allies and the Soviet Union soured and the Cold War began, the joint administration of Germany and Berlin broke down. Soon, Soviet-occupied Berlin and western-occupied Berlin had separate city administrations. In 1948, the Soviets tried to force the Western Allies out of Berlin by imposing a land blockade on the western sectors—the Berlin Blockade. The West responded by using its air corridors for supplying their part of the city with food and other goods through the Berlin Airlift. In May 1949, the Soviets lifted the blockade, and West Berlin as a separate city with its own jurisdiction was maintained.
Following the Berlin Blockade, normal contacts between East and West Berlin resumed. This was temporary until talks were resumed. In 1952, the East German government began sealing its borders, further isolating West Berlin. As a direct result, electrical grids were separated and phone lines were cut. The Volkspolizei and Soviet military personnel also continued the process of blocking all the roads leading away from the city, resulting in several armed standoffs and at least one skirmish with the French Gendarmerie and the Bundesgrenzschutz that June. However, the culmination of the schism did not occur until 1961 with the construction of the Berlin Wall.