The 'Domino Theory' was a Cold War-era belief that if one country in a region fell to communism, neighbouring countries would subsequently fall like a row of dominos. This theory was particularly influential in shaping US foreign policy in the mid-20th century, especially in relation to Southeast Asia. The fear was that if countries like Vietnam became communism, then other nations such as Laos, Cambodia, and Thailand would also become communist. This would spread Soviet and Chinese influence. The Domino Theory was used to justify American involvement in the Vietnam War, as part of a broader strategy to contain the spread of communism worldwide.
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