During the Cold War, the US proposed a theory that if one country fell to communism, neighboring countries would follow suit. Was this ever proven in the real world, or just an excuse for US imperialism that’s still prevalent long after the Cold War ended.
There's always more to be said, but you may be interested in /u/true_new_troll's answer to Why was the thinking behind "domino theory" wrong? as well as in /u/k1990 and /u/Itsalrightwithme's answers to What evidence do we actually have to prove the Domino Theory?