How did the domino theory shape U.S. policy toward Vietnam?

The domino theory was a cornerstone of U.S. foreign policy that posited that the fall of a single noncommunist state to communism would inevitably trigger the collapse of neighboring governments.1

Applied to Southeast Asia by President Dwight D. Eisenhower in the 1950s, this theory magnified the strategic importance of South Vietnam, regardless of its size. It served as a primary justification for the Kennedy and Johnson administrations to increase American military involvement in the Vietnam War. By framing the conflict as a critical stand against a chain reaction of communist takeovers, the theory ensured that U.S. policymakers felt compelled to prevent the loss of South Vietnam to maintain American credibility and prestige on the global stage. This belief system effectively locked the United States into a deepening military commitment, as officials feared that allowing any client state to fall would lead to the communization of the entire region.12