The USA took part in the Korean War for many valid reasons, the Domino theory probably being the most necessary reason. Eastern Europe was not the only place to collapse under communist ideologies during the cold war. In the far East, China also became communist (in 1949) which caused severe concern for the USA seeing as China has the largest population in the world. Like dominoes, Truman (US President at the time of the Korean War) believed that if Korea were to fall under Stalin’s (USSR leader at the time) sphere of influence then Japan would be next. Japan being a major trading partner with the USA caused Truman to react militarily to protect American trade.
Another reason for the United States to enter the Korean War was to undermine communism. Truman firmly believed that the capitalist, American way of life was in serious danger of being overwhelmed by the communist ideologies of the USSR. The battle of containing communism which was addressed in the earlier fight to oppress the USSR’s expansion into Greece and Turkey had developed a plan known as the Truman Doctrine which is what came into effect in the Korean War. Truman stated that the Truman Doctrine applied to Asia as well as Europe which is why military and economic aid was given to South Korea. Soon after Truman said help was to be given to South Korea, in April 1950, the American National Security Council issued the report NSC 68 which suggested that US forces disregard containment and begin rolling back the spread of communism. This recommendation led to the N. Korean oppression almost all of the way into China. Until soon after China helped N. Korea push UN and S. Korean forces back to the 38th parallel as it stands today.
Thirdly, Truman realized that US involvement throughout the world was crucial in the Cold War as USA was really in competition with the USSR for world domination. For this reason, the USA had to support S. Korea in order to fight the communist spread without directly attacking the USSR.
The USSR also had reasons of there own to enter the Korean War. Stalin like Marx believed the communist ideology should be spread all over the world to “free” people from Capitalism and other beliefs believed to be a waste of time. However, like Truman, Stalin did not want to be caught in a direct military confrontation with the US so in 1949 when Kim Il Sung (leader of N. Korea) approached him, he persuaded Stalin that the N. Korean army could liberate or conquer S. Korea in an attempt to spread communism. Stalin believed that the United States would not react to this invasion so he gave Kim Il Sung his agreement. Stalin also thought that an unopposed invasion of S. Korea would impose a major defeat on US authority in the entire world calling into doubt if they really are a worthy enough ally to compete with the USSR. Stalin was also determined to undermine NATO.
Stalin had many other substantial reasons to support the Korean War as well. From the beginning one of Stalin’s ambitions was to conquer the whole of Korea as it was established as one of the world’s most laudable industrial, agricultural, and transportation friendly complexes. And seeing as Korea was connected to the USSR via the Trans Siberian Railway, Stalin’s dream was to incorporate this region into his own. Fortunately for Stalin, he was able to attempt his vision of a united Korea under communism while staying in the background of the political scene while secretly supplying weapons and advice to his puppet leaders like Kim Il Sung.
The two “sparks” that directly led to the invasion of S. Korea by N. Korea include the boasting of Syngman Rhee (S. Korean leader – supported by the United States) that S. Korea would soon attack N. Korea. This in itself was a good enough reason for N. Korea to attack. Secondly, Dean Acheson, who was Secretary of State in the US at the time of the Korean War, stated to the Communist USSR that many countries were now under the protection of the USA and if the USSR attempted to “swallow” that regime behind its Iron Curtain, the US would comply with military assistance to that country. Unfortunately, Acheson did not include S. Korea as a protected state which gave N. Korea and the USSR another green light to attack.
All of the causes of the Korean War eventually come back to the Cold War’s superpowers annoyance for fighting in smaller proxy wars to make their opposing camps look like the least powerful ally of the two, and eventually succeed in world domination. Truman, after many years of struggle, comes out as the victor of the cold war, in that his aims to undermine communism, contain the communist spread, and fight to stay a world superpower have all been met with success and prosperity. Where as, Stalin’s dream to bring communistic ideologies to the rest of the world has failed.