Why did US get involved in Korean War? It was because of a variety of reasons.

The first is because of the domino theory. As China has just become communist in 1949 after its civil war where Mao’s army won Chiang Kai-Shek and forced him and his democratic followers out of China into Formosa (now Taiwan), it had gotten the Americans really worried. President Truman feared that the next domino or the next country that would fall under the hands of communism would be Japan if Korea was to fall to communism. Japan was a major trading partner of the US and the US feared that if she was to fall under the arms of communism she would lose a lot of her trade. The US was afraid that communism would spread all over the world, which seemed to be confirmed by the existence of COMINFORM. Furthermore, the US was very keen to make sure that the rest of Asia, especially Southeast Asia, would not fall under the arms of communism due to it being extremely close to China. Also, when China became communist, his administration was criticized for losing China to the communists by not giving the nationalists enough supplies for them to be able to defeat the communist forces and drive them out of China.

The second reason was because the US wanted to undermine communism. President Truman believed that capitalism, freedom and the American way of life were in the danger of getting overrun by communist ideas. The Truman doctrine and Marshall Plan were a few of the ways of containing the spread of communism in Europe but recently, the communists were catching up fast. The Soviets ended the American’s nuclear monopoly by successfully exploding a nuclear bomb in 1949 and China being communist in 1949 really worried the US. Thus in April 1950, the American National Security Council issued a report (NSC 68) which recommended the Americans to stop simply containing communism and take actions instead to “confront it with convincing evidence of the determination and ability of the free world to frustrate the Kremlin design of a world dominated by its will.” This caused Truman to think about driving the communists out of Korea. Thus when South Korea was invaded by the North Koreans, Truman took the opportunity to step in and intervene by defending a non-communist country from a communist invasion. Truman was concerned about the aggressive nature of the communist.

In his speech of June 27, Truman argues that "communism has passed beyond the use of subversion to conquer independent nations and will now use armed invasion and war." His statement shown that he believed the invasion of Korea was just a small part of a much bigger plan by the newly communist China and possibly Russia as well. He believed and told his advisors that the invasion of Korea was inspired by the Soviet Union. He also said that America had to step up and fight now otherwise there would be no telling as to what the communists would do. Truman was concerned about the future of the non-communist governments currently in Asia and he pledged to defend several countries including Formosa from communist attacks as well as to support the French who were in Indochina. Also, his statement reflected a new order in the American military system. Even though the US was the one taking the lead in the Korean War, it did it under the rubrics by the United Nations. He also reminded all of the members of the United Nations to "consider carefully the consequences of this latest aggression in Korea" and that America "will continue to uphold the rule of law."

Furthermore, Truman realized that the US and Russia were both competing for world domination. If America were to step into the Korean War, she would be able to indirectly attack Russia without getting into a hot war as the effects of both parties getting into a hot war was undesirable and they would rather fight a proxy war where the US controls South Korea and Russia controls North Korea.

Another reason for entering the Korean War was to save face. Korean was not important to the US economically and thus the US had no need for Korea. However, it entered partially because it did not want to seem weak and cower before communism. Thus, the US brought its forces in to defend Korea so that would appear strong to the rest of the world.

In conclusion, the US decided to intervene in the Korean War because of the domino theory, to try to undermine the spread of communism to protect the American way of life, to compete with Russia indirectly without starting a hot war as well as to appear strong and save America’s face.


References:
http://www.archives.gov/education/lessons/korean-conflict/
http://www.sparknotes.com/history/american/koreanwar/context.html
http://www.korean-war.com/Archives/2001/04/msg00014.html