Final answer:
The United States entered the Vietnam War to stop the spread of communism.
Step-by-step explanation:
The United States entered the Vietnam War because they wanted to stop the spread of communism. The main reason for America's involvement in Vietnam was the policy of containment, which aimed to prevent the spread of communism beyond its current borders. The United States feared that if South Vietnam fell to communism, other countries in Southeast Asia would follow suit, leading to a domino effect.
Learn more about reasons for the United States' involvement in the Vietnam War