Throughout the long period of American involvement in Vietnam, Washington officials often justified US intervention by referring to the domino theory. Even before President Dwight D. Eisenhower formally articulated the theory in 1954, civilian as well as military analysts had set out a version of the theory, linking the outcome in Indochina to a chain reaction of regional and global effects. Defeat in Vietnam, they warned, would have calamitous consequences not merely for that country but for the rest of Southeast Asia and perhaps beyond. Over time, US officials moved to a less mechanistic, more psychological version of the theory. Credibility was the new watchword, as policymakers declared it essential to stand firm in Vietnam in order to demonstrate American determination to defend its vital interests not only in the region but around the world. But it was not only American credibility on the world stage that mattered; also at stake, officials feared, was their own and their party’s credibility at home. This chapter examines these permutations of the domino theory, with particular focus on the crucial 1964–5 period under Lyndon B. Johnson.