A domino effect is the cumulative effect produced when one effect sets off a chain of similar effects. It is used as an analogy to falling row of dominoes. The Domino theory was used by the US administrations during the Cold War. It speculated that if one country in the region fall under the communist regime, then the surrounding countries would follow it. President Harry Truman declared the Truman Doctrine in 1947. By the Doctrine US would aid countries who are fighting communism ( at that time Greece and Turkey ). Truman wanted to stop the effects of the Domino theory.