What was the Domino Theory as noted in the nonfiction book, Most Dangerous?

Adherence to the Domino Theory was an important part of American foreign policy during the Cold War when the spread of communism was considered a real threat to democracy. The theory held that countries were like dominos standing on end and when one fell to communism it would fall into the domino in front of it and make it fall and so on. The fear of the domino theory taking effect in Vietnam, it was felt, put other Indochinese countries like Laos and Cambodia at risk of becoming communist if Vietnam fell.