After the end of the Second World War the two war time allies THE USA and SU became involved in a war of ideologies the cold war. The US saw communism as a threat to democracy and capitalism. Therefore the US set out a new foreign policy that was of containment of communism in the Truman doctrine. One of the main reason why the us go involved in the Korean war was because of the Truman doctrine basically guaranteed the US supporting countries fighting communism. The first one being Korea. Also it was to keep the Soviet Union from spreading communism. it was believed that it would be a domino effect if one country fell to communism.
Trumnan had also given a speech that he would go to any lengths to counter aggression it was more a UN effort with the United States leading the way. It started in June or July 1950 and ended in a stalemate 3 years later at the 38th parallel. A cease fire was proclaimed but no peace treaty was ever forged or signed, so technically the two Koreas are still in a state or war. . It also proved the power of U.S. against the communist.

Harry Truman had given a speech where he warned the communists that we would go to any lengths to counter aggression in...and he named a number of countries (Greece and Austria among them). Absent from this list was Korea, not because Harry didn't care about that peninsula, but because he was using a brief list of exemplars. it was merely another episode in the ongoing Cold War between these two super-powers. On the surface, the Korean War seemed to be a war between South Korea and North Korea, but in actuality the superpowers were using it as a front to combat each other without actually going into a 'hot war' which - as both had the atomic bomb - would have been a scenario of mutually assured destruction.
The USA went to war in Korea for three main reasons. The first reason was the 'Domino theory'. Salami tactics in Eastern Europe was not the only place where Communists were coming to power. In the Far East, too, they were getting powerful - China turned Communist in 1949. Truman believed that, if one country fell to Communism, then others would follow, like a line of dominoes.
. President Truman believed that capitalism, freedom and the American way of life were in danger of being overrun by Communism. The Truman Doctrine had been one of 'containment' - stopping the Communists gaining any more territory. In April 1950 the American National Security Council issued a report (NSC 68)