After World War II, the United States and the Soviet Union were the only two superpowers that remained. This post World War period was referred to as the "Cold War".
Throughout the World War I and II period an "anti-balance" of power system developed worldwide. As no single superpowers emerged, many believed that the survivors or winners of World War II would become those military superpowers. They were correct, the United States and the USSR were left dominant, as an "bipolar system" emerged. This system would lock both superpowers into frantic war that would drain both of them. Throughout the Cold War, the United States self interests became more important and crucial than ever before.
It was for their best self-interest to contain communism from spreading in Southeast Asia. The Americans ideals and morals were focused on winning the Cold War no matter the cost. Communism had already began to expand as the Soviet Union's hegemony over East Europe turned the majority of those nations into communism. As one European nation after the other fell under communism it was for the United States best interest to stop it. After China fell victim to communism, the United States was afraid that communism would keep on spreading in Asia as it did in Eastern Europe. The USfirst became involved in Vietnam because American policymakers believed that if the entire country fell under a Communist government, Communism would spread throughout Southeast Asia. This is an example of Eisenhower's "domino theory".
This theory can be the basis of American self-interest because this theory supports Truman's containment policy. If you line up many dominoes in a straight line, and then you push thefirst domino it will be just a matter of time before the other dominoes fell. If Vietnam were to fall, it will be just a matter of time before other Southeast Asian countries fell to communism. Vietnam had .