The domino theory is a geopolitical theory which posits that increases or decreases in democracy in one country tend to spread to neighboring countries in a domino effect. It was prominent in the United States from the 1950s to the 1980s in the context of the Cold War, suggesting that if one country in a region came under the influence of communism, then the surrounding countries would follow. It was used by successive United States administrations during the Cold War as justification for American intervention around the world. U.S. President Dwight D. Eisenhower described the theory during a news conference on April 7, 1954, when referring to communism in Indochina as follows:

Finally, you have broader considerations that might follow what you would call the "falling domino" principle. You have a row of dominoes set up, you knock over the first one, and what will happen to the last one is the certainty that it will go over very quickly. So you could have a beginning of a disintegration that would have the most profound influences.