Answer:
The American effort to contain communism was the main reason why the U.S. got involved in the Vietnam War.
Explanation:
After World War II, the United States and the Soviet Union became the two superpowers of the world. The U.S. represented western democracy and capitalism, and the Soviet Union represented one-party rule and communism.
The two countries competed for global influence, and for this reason, the United States was wary of the possible spread of communist ideas, and the growth of the Soviet Sphere.
When a communist insurgence emerged in Vietnam, the U.S. government became paranoid that it would spread to several neighboring countries like Camboya or Malasyia, in a geopolitical doctrine known as the dominio effect. This was the main reason why the U.S. got involved in the Vietnam war, in order to stop the spread of communism in Vietnam, and thus, the possible spread to other countries.