the domino theory
noun [ C usually singular ] politics specialized uk/ˈdɒm.ɪ.nəʊ ˌθɪə.ri/ us/ˈdɑː.mə.noʊ ˌθi.ə.ri/
多米诺骨牌理论（1950和1960年代的政治用语，指如果一个国家成为共产主义国家，其周围国家也更易成为共产主义国家）
the belief, especially in the 1950s and 1960s, that if one country became communist, other countries near that country would more easily also become communist