Historians have settled on a two-part answer to why Stalin blockaded West Berlin in June 1948. The immediate trigger was the Western decision to introduce the Deutsche Mark in their zones, including in West Berlin, which Moscow read as the prelude to a separate West German state. The deeper cause was Stalin’s conviction — drawn from the Western withdrawal from Iran and from the Red Army’s position in central Europe — that the Americans and British would back down rather than risk war.
Three pressures converged in the spring of 1948. First, the Western allies had decided to fuse their three German zones into a single economic unit, the Bizone, and then into a Trizone with the French. Currency reform followed on June 20: the Deutsche Mark replaced the inflated Reichsmark in the western zones, including the three western sectors of Berlin. Stalin saw this as a deliberate prelude to a West German state that would be permanently aligned against him.
Second, Stalin feared the impact of the Marshall Plan, announced by Secretary of State George Marshall in June 1947, on Eastern Europe. The offer of American economic aid to a continent ravaged by war was, in the Soviet view, an instrument of American penetration. Moscow had already forced Czechoslovakia and Poland to refuse participation. The Deutsche Mark looked like the next move.
Third, the Czechoslovak coup of February 1948 had pushed the Cold War to a new pitch. Stalin felt the West was retaliating through Germany; he decided to retaliate through Berlin. Cutting off West Berlin by land was, he calculated, the lowest-risk option that would still test American resolve. He expected the city to fall in a few weeks.
The gamble failed. Truman chose to fly, not fight and not withdraw. Eleven months later, the Soviets quietly lifted the blockade. The episode, treated in full in the Berlin Blockade and Airlift article, is the founding episode of the Cold War in Europe.