The domino theory was a Cold War idea in the United States that if communists took over one government in a region that communism would spread into neighboring countries—each country falling to communism like dominos lined up in a row.
American President Dwight D. Eisenhower (1890 – 1969) used the theory to justify American support for non-communists in the former French colony of Vietnam in a speech in 1954. He said,
The domino theory resulted in America using the policy of containment to resist the expansion of communism. The Americans did this by
using its military to fight communist movements and governments in places like Vietnam
supplying money, weapons, and political support to anti-communist groups who resisted communist expansion in their countries.