Soviet and American soldiers who had met at Berlin in 1945 quickly became representatives of two separate worlds. The borders drawn at Yalta and Potsdam had left Eastern Europe in Moscow's sphere; in March 1946, speaking at Fulton, Missouri, Churchill described an "Iron Curtain" stretching from the Baltic to the Adriatic. In 1946-47 the Greek civil war intensified, the USSR pressed for influence over the Turkish Straits, and the Iran crisis flared. The war was over, but peace had not been built.

On 12 March 1947 President Truman addressed Congress to request 400 million dollars in aid for Greece and Turkey; his line that "it must be the policy of the United States to support free peoples who are resisting attempted subjugation by armed minorities or by outside pressures" became known as the Truman Doctrine — the first comprehensive frame of the Cold War. Three months later, on 5 June 1947, Secretary of State George Marshall announced the European Recovery Program at Harvard: between 1948 and 1952 some 13 billion dollars — roughly 150 billion in today's terms — flowed to 16 countries. Stalin saw the Marshall Plan as a threat to the Soviet sphere and forbade his satellites to join. In September 1947 the USSR founded Cominform (the Communist Information Bureau); Andrei Zhdanov's "two camps" speech drew the front lines explicitly.

Within a short time, the world began to freeze into two military and economic blocs. In June 1948 the Soviets cut Western road and rail access to West Berlin; the United States and Britain sustained the city through 11 months of the Berlin Airlift, with more than 270,000 flights — in May 1949 the Soviets lifted the blockade. That same month the Federal Republic of Germany was founded; in October, the German Democratic Republic. NATO was born in April 1949; the Warsaw Pact answered in 1955. Over the next forty years the rivalry never became direct war but the stage was shared by proxy wars in Korea (1950-53), Vietnam (1955-75) and Afghanistan (1979-89), by the Cuban Missile Crisis (1962) and by the space race (Sputnik 1957, the Moon 1969).

The question of "who started the Cold War" is one of the oldest debates in historical scholarship. The traditional (orthodox) school of the 1950s placed primary blame on Stalin's expansionism; in the 1960s revisionists such as William Appleman Williams emphasised American economic expansionism and the diplomatic use of the atom bomb; from the 1970s onward John Lewis Gaddis and other post-revisionists stressed that the mutual threat perceptions of both sides produced an almost inevitable spiral. The common view today is that it cannot be reduced to a single cause or a single culprit; two competing ideologies replaced two retreating armies without leaving any vacuum. On the Eon timeline 1947 is the year in which the political map of the second half of the 20th century was drawn.