The United States pursued a policy of containment against the spread of communism following World War II. This led to the origins of the Cold War between the US and Soviet Union. The Truman Doctrine and NATO alliance in Europe were early Cold War policies to counter Soviet expansion. The US fought wars in Korea and Vietnam to prevent the spread of communism in Asia according to the containment policy. At home, the Cold War threat of nuclear war and fears of communism affected American society.