or domino effect
Doctrine of U.S. foreign policy during the Cold War , according to which the fall of a noncommunist state to communism would precipitate the fall of other neighbouring noncommunist states.
The theory was first enunciated by Pres. Harry Truman , who used it to justify sending U.S. military aid to Greece and Turkey in the late 1940s. Dwight D. Eisenhower , John F. Kennedy , and Lyndon B. Johnson invoked it to justify U.S. military involvement in Southeast Asia, especially the prosecution of the Vietnam War .