Occupation zones after 1945. Berlin is the multinational area within the Soviet zone
The Berlin Blockade was from 24 June 1948 to 11 May 1949. It began when the Soviet Unionblocked railway and road access by the three Western powers (the Americans, British, and French) to the Western-occupied sectors of Berlin. The Blockade stopped after the Western powers used airplanes to airlift food and other things that people needed.
The Soviet Union began the blockade because they thought that monetary reform in the three German Occupation zones controlled by the Western powers which started on 21 June 1948 made the western parts of Germany too strong and wanted to force the west out of their occupation zone. The Russians wanted one Germany, without an army, that they could control.
When the Allied Powers met at Potsdam they agreed to divide Germany into four occupation zones, and divided Berlin into four zones as well. Berlin was surrounded by the Soviet occupation zone, so the only way to get to the three western controlled sectors was driving through Soviet-controlled territory.