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According to the domino theory, neighboring nations can be vulnerable to the spread of communism because of their cultural similarities.
During the Cold War years, the Domino Theory referred to the idea of the United States that one Cumminist nation could be led to the spread of Communism in another neighboring nation, and another, like dominos that were aligned. That thinking led to the intervention of the United States Army in Vietnam. During these Cold Years, the Soviet Union wanted to spread Communist in many countries and the United States wanted to stop it.
In simpler words, the domino theory was based on the idea that a communist victory in one country could lead to the spread of communism to other countries.