Why did the United States military get involved in Vietnam

This ones a bit tricky, its called the domino effect, The United States thought if Vietnam fell under communist control, then other nations around that would fall to communism, like domino, the United States didn't want that, so they sent over troops, thus creating one of our biggest mistakes we ever made.

Fear that Communism would take over Southeast Asia

Explanation:

Apex

Sorry if I'm wrong.