The United States had been involved in Vietnam since 1954, when the French withdrew their colonial occupation and the Soviet Union and the United States agreed to split the new nation (Laos and Cambodia also came into existence at the time, all from "French Indochina") until elections could be held to decide on a national government. After the United States realized that Vietnam would vote a Communist government into power, however, it reneged on the deal. A war broke out as the North Vietnamese Communist leader Ho Chi Minh attempted to unite the nation. The United States gave aid to South Vietnam for years, and was very involved in its internal politics (JFK allowed a coup, for example). Finally, when it became clear that the United States would either have to escalate its involvement or watch the North Vietnamese win, LBJ decided to escalate the war in 1965. The goal was to defeat the North Vietnamese army as well as its South Vietnamese ally the Vietcong, which the United States mistakenly viewed as a Communist puppet and not as a nationalist organization seeking to finally remove Western influence over the nation. LBJ and others believed that if South Vietnam fell to Communism, other nations might fall to Communism too. This so-called "domino theory" can be seen in LBJ's speech he gave justifying the escalation of American involvement in the war. Leaving South Vietnam to its fate, LBJ explained, would not "bring peace, because we learned from Hitler at Munich that success only feeds the appetite of aggression." Just as Hitler had continued his conquests even after Munich, so too would the Communists continue their conquests in the region, LBJ argued.

And indeed, after the United States finally withdrew in 1973, both Laos and Cambodia fell to Communism. But by the end of the decade, regional tensions had risen to the point that Vietnam invaded Cambodia. Communism did not prove to be some monolithic power that could spread from state to state unchecked. Thailand, which was often cited as a potential target for future expansion, never actually fell to Communism.