At the Potsdam Conference in August 1945, after Germany's unconditional surrender on May 8, 1945, the Allies divided Germany into four military occupation zones – French in the southwest, British in the northwest, United States in the south, and Soviet in the east. The intended governing body was called the Allied Control Council. The commanders-in-chief exercised supreme authority in their respective zones and acted in concert on questions affecting the whole country. Berlin, which lay in the Soviet sector, was also divided into four sectors with the Western sectors later becoming West Berlin and the Soviet sector becoming East Berlin, capital of East Germany.
In 1945 and in the beginning of 1946 there were many stamp-issuing entities in Germany. In 1946 USA, Britain and Soviet Union agreed on using same stamps in territories under their administration. Here are some of them:
By early 1946, the Western Allies (the United States and Britain in particular) had become convinced that Soviet expansionism had to be contained. The Soviet Union's seizure of Polish territory and the drawing of the Oder-Neisse border (which gave formerly German territory to Poland) and its antidemocratic actions in other countries occupied by Soviet forces, persuaded Western leaders that the Soviet Union was aiming for communist domination of Europe. Churchill's use of the expression "Iron Curtain" to describe the Soviet cordoning off of a sphere of influence in Europe illustrated a basic change in attitude toward Soviet intentions on the part of Western leaders. Therefore, on January 1, 1947, The Bizone, consisting of the United States and British zones, was proclaimed.
In the spring of 1949, the French occupation zone also joined the Bizone, creating the Trizone. On May 25th, 1949 Federal Republic of Germany was formed and German stamps replaced occupation issues.
But that was not the case in West Berlin. West Berlin was surrounded entirely by the Soviet sectors and so was an enclave. The western allies were guaranteed an air corridor, but not road or rail access. Although West Berlin was de facto part of West Germany, it was not considered to be a Bundesland, nor part of one, and the Constitution had no application there. Instead, it was administered by the West Berlin Mayor and city government. One of the anomalies was West Berlin postal administration, separate from West Germany's, which issued its own postage stamps until 1990.