The US battled communism in Asia in the 1950s by using the Domino Theory as a justification of its support to a non-communist regime of South Vietnam against North Vietnam's communist government. Because of this, the US became a part of the Vietnam War.
The Domino Theory states that the fall of IndoChina to communism will also lead other countries to fall and become under communist control. The US are against communism, that is why they used the Domino Theory to support their stance.
