The united states used the to justify its intervention in southeast asia.

During the Cold War there was a concern in the United States over the expansion of communism. This fear of communism manifested itself in the domino theory an idea which governed much of us foreign policy starting in the 1950s. The domino theory was essentially the idea that if one nation fell to communism its neighboring counties would as well and this process would repeat itself almost resembling a row of dominos falling (hence the name). Because of this theory many political leaders in the us feared that if former French colonies like Vietnam fell to communism than their neighboring counties would as well including Laos, Cambodia, etc.