The United States fought in the Vietnam War to prevent the spread of communism.
Containment . It is best known as the Cold War policy of the United States and its allies to prevent the spread of communism . Vietnam War . The North Vietnamese army was supported by the Soviet Union , China and other communist allies and the South Vietnamese army was supported by the United States , South Korea , Australia , Thailand and other anti-communist allies . The U.S. government viewed its involvement in the war as a way to prevent a communist takeover of South Vietnam . This was part of the domino theory of a wider containment policy , with the stated aim of stopping the spread of communism worldwide .