What was a key reason for the U.S. involvement in the Vietnam War?

The primary reason for U.S. involvement in the Vietnam War was indeed to prevent the spread of communism in Southeast Asia. This was rooted in the broader context of the Cold War, during which the United States adopted a containment policy aimed at stopping the expansion of communist influence worldwide. The fear that the fall of Vietnam to communism would lead to a "domino effect," causing neighboring countries to also fall to communism, significantly influenced American foreign policy decisions.

By intervening in Vietnam, the U.S. aimed to support the non-communist government in South Vietnam and ensure that communism did not take hold in the region. This commitment was reflected in increased military and financial aid to the South Vietnamese government and ultimately led to direct military involvement when the conflict escalated during the 1960s. Hence, the focus on containing communism fundamentally motivated the United States' decisions regarding its engagement in Vietnam.