Domino theory is a military theory which was coined by President Dwight D. Eisenhower during the Cold War. Eisenhower believed that the fall of certain communist countries could create a "domino effect" on the international stage, toppling other communist countries as a result. This theory was also used to describe the opposite effect. Eisenhower believed that if one nation or region came under the control of communism that communism would spread and lead to the spread of communist governments popping up all over the world. If the dominoes fell one way the world would be saved from communism while if they fell the other way the world would be overtaken by communism.
Eisenhower's reasoning for referring to this spread or decline of Communist states as the Domino Effect is due to the fact that he believed that if Communists succeeded in taking over Indochina they would have the materials, resources and human support to succeed in taking over many other countries in the area. In turn, he also felt that if they failed at taking over Indochina the exact opposite would happen and other countries would not believe in the strength and power of communist regimes.
The domino effect was used by Eisenhower and subsequent United States Presidents John F. Kennedy and Lyndon Johnson to justify military intervention in certain areas. The domino theory has never been conclusively proved or disproved and political theorists continue to debate the merits of the theory to this day. There are many examples where the installation of communist governments did not incite a spread of communism to other nations in the area and there are also examples where communism did spread.
In the world today some politicians still refer to the domino effect when making policy decisions. Prior to the 2001 invasion of Iraq, some conservative lawmakers argued that the installation of a Democracy in Iraq could lead to a domino effect in the Middle East, with other countries implementing a Democracy after seeing it operate successfully.