What is the name of the theory that if one country falls to communism, others will as a result?

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The Domino Theory was a Cold War-era belief that if one country fell under communist influence or control, neighboring countries would subsequently fall like a row of dominoes. This theory was used by successive U.S. administrations to justify American involvement in Vietnam and support for non-communist regimes in Southeast Asia.

What Is the Name of the Theory That If One