The Marshall Plan’s manifold effects included the hardening of the division of Europe, the movement for integration within western Europe, and the creation of the two Germanies. “Bizonia,” the product of an economic merger between the U.S. and British occupation zones, was announced on May 29, 1947, and a new U.S. policy followed on July 11 that ended Germany’s punitive period and aimed at making its economy self-sufficient. When in March 1948 some of the western European states responded to the coup in Czechoslovakia by signing the Brussels Treaty and pressing ahead with the establishment of a West German currency and government, the Russians walked out of the Allied Control Council. On June 24, Soviet occupation forces in the eastern zone blocked Allied road and rail access to the western zones of Berlin. This first Berlin crisis, made possible by the anomaly of a U.S.-British-French interest 100 miles inside the Soviet zone, forced Truman to define the limits of his “get tough” policy. Clay and Acheson advocated sending an armed convoy along the access routes to assert Allied rights, but neither the Joint Chiefs nor the British and French were prepared to risk war. Instead, the United States responded with an enormous airlift, totalling 277,264 sorties, to keep western Berlin supplied with food, fuel, and medicine. Perhaps Stalin hoped to drive the Allies from Berlin, or to prevent the setting up and possible rearmament of a West German state, or to induce the American electorate in 1948 to return to isolationism. In the event, the blockade only frightened the Western powers into stronger new measures. On April 4, 1949, the foreign ministers of the United States, Britain, France, Belgium, the Netherlands, Luxembourg, Italy, Portugal, Denmark, Iceland, Norway, and Canada founded the North Atlantic Treaty Organization (NATO) in Washington, D.C., providing for mutual aid in case of attack against any member. On May 8, the West German parliamentary council adopted a constitution, and on May 23 the Federal Republic of Germany came into being. Stalin acknowledged defeat in Berlin and lifted the blockade on May 12, but the Soviets countered by creating mirror institutions—the German Democratic Republic (October 7, 1949) and the Council for Mutual Economic Assistance (Comecon) in the Soviet bloc.

The parallel and hostile German states and regional alliances institutionalized and militarized the Cold War even as the Communist ideological offensive and the Truman Doctrine had universalized it. Before this first phase of the Cold War closed, however, two events called into question root assumptions of the two sides. The first was the West’s assumption that Communism was a monolithic movement controlled from the Kremlin. In June 1948 the world became aware of a rift between Stalin and Tito that threatened to shake the Soviet empire of “people’s democracies.” This rift could be traced to the war, in which Tito’s Communist partisans had expelled the Nazis from Yugoslavia without large-scale aid from the Soviet Union. As a national hero, Tito had strong domestic support and thus was not personally dependent on Stalin. He even persevered in support for the Greek Communists while Stalin was adhering to his 1944 agreement with Churchill to keep hands off Greece. When Stalin and Molotov vetoed his plans for a Balkan confederation, Tito purged Yugoslav Communists known to be in the pay of Moscow. Stalin countered with brutal threats and a purge of Communists in the satellites accused of Titoist tendencies. But Tito held firm: Yugoslavia would “choose its own path to Socialism,” seek economic ties with the West, and indirectly place itself under Western protection. Tito also ceased to support the Greek Communists, and the civil war there soon ended in a victory for the royal government (October 1949).

The second assumption of the early Cold War was shattered in August 1949 when the Soviet Union exploded its first atomic bomb. Its development might have been hastened by espionage, but Soviets had been among the leaders in nuclear physics before the war, and knowledgeable observers had known that a Soviet atomic bomb was only a matter of time.