During the Cold War, the "domino theory" dominated American foreign relations. The theory proposed that a communist takeover over of one country would quickly lead neighboring countries to fall to communism, like dominoes falling in succession. Cold War foreign policy was enveloped in the domino theory, which led to policies like containment, the Marshall Plan and the wars in Korea and Vietnam. Even after the collapse of the Soviet Union, a different kind of containment -- aimed towards the Middle East rather than communism -- continued in American foreign affairs.

Containment

The domino theory led to one clear conclusion: limiting the spread of communism was essential to preventing it from going viral. As early as the late 1940s, President Harry Truman initiated a policy of containment to try and contain communism where it already existed, and prevent its spreading elsewhere. Truman, for example, gave aid to Turkey and Greece to prevent civil unrest that he feared might turn into communism. Likewise, Truman authorized financial support to the French in their war with communist-leaning Indochinese rebels. President Dwight Eisenhower, however, was the first to publicly call this policy a "domino theory," when he spoke about the importance of preventing communism's rise in Indochina.

Korean War

The Korean War was the first major military intervention that resulted from the domino theory. In 1945, Korea was split into two separate societies. The north was communist while the south was free and democratic. When North Korea invaded South Korea in 1950, with the backing of communist countries like the Soviet Union and China, the United States saw the domino theory materialize in Asian. One communist nation -- supported by other communist countries -- was invading and trying to spread communism in a free country. With the support of the United Nations, the United States went to war with North Korea. After difficult fighting, the conflict ended with a stalemate. Communism hadn't spread further, but it wasn't defeated, either.

Vietnam War

Though U.S. involvement with Vietnam began under President Eisenhower, American intervention escalated during the Kennedy and Johnson administrations. North Vietnam had fallen to a communist regime, while South Vietnam was controlled by the anti-communist Ngo Dinh Diem regime. When Diem was murdered in a coup in 1963, the United States reaffirmed its commitment to the domino theory and offered aid to the new South Vietnamese government to prevent North Vietnam's communism from expanding into the South. By the later 1960s, President Lyndon Johnson turned the Vietnam conflict into a full-scale war and committed around half a million U.S. troops. The Vietnam War, however, quickly became unpopular in the United States, and failure to win made many Americans question the legitimacy of the domino theory.

Containment Today

Though the domino theory is most closely associated with the Cold War between the late 1940s and early 1990s, some foreign policy analysts believe it continues today. The U.S. invasion of Afghanistan -- and prolonged presence in the country -- stemmed from a belief that the Taliban was expansionary and would spread into neighboring countries unless the U.S. attacked it in Afghanistan. Similarly, U.S. foreign policy regarding the Iranian nuclear program suggests domino theory influence. Many foreign policy officials believe Iran's attainment of a nuclear weapon would cause other countries in the region to create their own nuclear bombs. This is similar to the belief that communism in one country would spread elsewhere.