After the US's World War II alliance with the Soviet Union dissolved, the Soviets resumed their push to establish communism in other countries around the world. In Eastern Europe, the US had little choice but to watch this happen, as that was territory that had been occupied by the Soviets in the process of defeating Germany. The Soviets soon built their own atomic bomb, and their new belligerence was symbolized by Khrushchev's famous threat: "we will bury you." Meanwhile, the Chinese Civil War ended in 1949 with Communists in control of that huge landmass and population, and the cry "who lost China?" was much heard in American politics.

The end of colonialism in Southeast Asia meant the rise of Communists, among other political parties, in several countries. But they were particularly strong and successful in Vietnam. In hindsight, we've come to laugh at American officials who believed the "domino theory"—that if Vietnam went communist, Thailand or even Japan or the Philippines might follow, threatening American military bases and interests in the region. But this wasn't an entirely baseless fear at the time. As it was now clear that the US would be in a permanent standoff with the Soviets, it was thought imperative that the USSR not have puppet states on both ends of the Eurasian continent.

Two books I've found useful in learning about this period are The United States and the Origins of the Cold War by John Lewis Gaddis; and for a more personal, inside perspective, Counsel for the President, by Clark Clifford.