What is the domino effect?

The belief was that a fall of a non-communist state would precipitate the fall of the next one and then another, until they all fall like dominoes. Karl Marx figured this would work for him. American presidents used this theory in places like Vietnam, Turkey, and Greece, as examples of countries they could save from communism successively falling to democracy, like dominoes. In Animal Farm, other farmers around Mr. Jones's failed farm felt threatened that their farms would fall to animalism as well, like dominoes.