In the years following the conclusion of World War II (1939–45), the nations of Western Europe and the United States became alarmed by Soviet advances into Eastern Europe, and many Europeans and Americans voiced concerns that communists, led by the Soviet Union, were plotting to take over the world. Political leaders in England, the United States, and elsewhere referred to this new menace in grim terms. In March 1946 former British prime minister Winston Churchill (1874–1965) warned of an “Iron curtain” of Soviet totalitarianism that had divided the European continent, and in 1947 U.S. president Harry S. Truman (1884–1972) announced a policy of containment of communist incursion into other countries. This policy came to be known as the Truman Doctrine, and it remained an integral part of American foreign policy for the next 40 years, ultimately leading to the nation’s involvement in the Korean War (1950–53) and the Vietnam War (1954–75).

The eroding relationship between the Western powers and the Soviet-led countries of Eastern Europe was largely brought on by disagreements over Germany. At the close of World War II, marked differences of opinion on what to do with Germany had resulted in a plan for joint government of the nation by the Allies—the Soviet Union, the United States, Britain, and France. But the arrangement quickly proved unworkable. By 1948 Germany was in serious economic straits, and the United States, Britain, and France began to discuss uniting their zones. The Soviets responded by ordering a blockade of land and water traffic into Berlin, control of which had been divided between the Allies after the war (the Soviets controlled East Berlin, while the other Allies controlled West Berlin). To counter the blockade, Great Britain and the United States ordered an airlift operation to provide food and other supplies to the people of West Berlin, alleviating the effects of the 11-month Soviet blockade. In 1949 the East-West differences resulted in the formal division of Germany into two countries: West Germany, formed by the zones occupied by the United States, Great Britain, and France, was allowed to form a democratic government, and it became officially known as the Federal Republic of Germany. The same year, East Germany (also known as the German Democratic Republic) was formed out of the Soviet zones and was folded into the “Eastern bloc” countries.

By 1949, the year that the Soviet Union exploded its first nuclear bomb, the world had been roughly divided into two camps: the United States and its democratic allies, which included the nations of Western Europe and other anticommunist governments; and the Eastern bloc, the Soviet Union and its satellite countries. These camps were soon girded by formal political alliances. In 1949 a military alliance known as the North Atlantic Treaty Organization (NATO) was formed by 12 nations (the United States, Great Britain, France, Italy, Norway, Portugal, Iceland, Denmark, Canada, Belgium, Luxembourg, and the Netherlands). By 1955 three more countries—Greece, Turkey, and West Germany—joined the alliance. The Soviet Union responded by creating the Council for Mutual Economic Assistance (COMECON) in 1949 and the Warsaw Pact in 1955. COMECON was an effort to coordinate economic and industrial activities among Communist nations, while the Warsaw Pact was a military agreement between the Soviets and the Communist governments of Eastern Europe. The Cold War was on.