Dominoes
One of the most widely used explanations for why the United States had to be involved in Vietnam during the 1960s/70s was that communism had to be stopped in its tracks, that if communists were allowed to “take over” Vietnam the rest of South East Asia would quickly follow. It was called the Domino Theory to suggest the image of how a row of domino tiles, placed on their edges and near to each other, could all be made to topple over by simply knocking the first one over onto the second.
