What did the cold war have to do with The Domino Theory?

The Domino Theory is the term given to the "Wests" fear of the spread of Communism, it was believed at the time that if one Asian Country like Vietnam for instance were to fall to "The Communists" that the country next to it would also likely fall and the cycle would continue until the whole world was engulfed in communism, just like a Domino effect.

The Truman Doctrine which was announced in a speech by President Harry S. Truman in 1947 would become the basis for America's foreign policy during the Cold War. Even though the speech originally called for the "West" to support Greece and Turkey in their resistance against the communist regimes trying to gain power in their own countries. This doctrine stated that America would help any nation resisting Communism. How does this tie in with the Cold War and the Domino theory? It's simple, as the Cold War was a battle between Ideology's (Democracy vs. Communism) America set out to stop the spread of communism, especially in Asia, in places like Vietnam (Stopping the Domino Theory from ever becoming a reality). This therefore created conflicts such as the Vietnam War which were part of the global "Cold War" period.

Hope it makes sense, was rather tired while typing this :/