domino theory  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.  Learn more about
domino theory with a free trial on  This article is about chain
reactions. For the political theory, see Domino Theory.
Demonstrations of the effect  The classic demonstration involves
setting up a chain of dominoes stood on end, and toppling the
first domino. That domino topples the one next to it, and so on.
In theory, however long the chain the dominoes will still fall.
This is because the energy required to topple each domino is
less than the energy transf