Containment was a foreign policy strategy followed by the United States during the Cold War. First laid out by George F. Kennan in 1947, Containment stated that communism needed to be contained and isolated, or it would spread to neighboring countries. This spread would allow the Domino Theory to take hold, meaning that if one country fell to communism, then each surrounding country would fall as well, like a row of dominoes. Adherence to Containment and Domino Theory ultimately led to US intervention in Vietnam, as well as in Central America and Grenada.
Containment and Domino Theory as applied to Southeast Asia:
If communism were not contained in North Vietnam, then South Vietnam, Laos, Cambodia, and Thailand would inevitably become communist as well.