After World War II, allies who had fought fascism together became rivals for world power. From 1945 to 1990, there was a period of tension called the Cold War. Capitalist countries, led by the US, clashed with the communist countries of the Soviet Union and China.
Both sides in the Cold War were heavily armed with nuclear weapons. In 1962, the Soviet Union secretly stationed missiles in communist Cuba. The US found out and demanded that the missiles be removed. The Soviet Union gave in, and the world narrowly avoided a nuclear war.
In 1954, the French colonial army in Vietnam was defeated by communist rebels. The country was divided into North and South Vietnam, and the US intervened to support an anticommunist government in the South. During the 1960s, troops were sent to fight the communists. They failed to defeat them, and Vietnam united under communist rule in 1975.
World War II had left communist governments in control of central and eastern Europe. They were opposed by the nations of western Europe and the United States. The two hostile sides, or “blocs,” became isolated from each other. In a speech, British politician Winston Churchill said that it was as if an “iron curtain” had fallen across Europe.