For what reason did the United States primarily become involved in Vietnam ?

Communist takeovers

Explanation:

Im not 100% sure on that tho

the name of the theory is landmass theory.

roosevelt believed that in light of the country's recent military successes, it was unnecessary to use force to achieve foreign policy goals, so long as the military could threaten force. this belief, and his strategy of “speaking softly and carrying a big stick,” shaped much of roosevelt's foreign policy.