The involvement of the United States in the Korea War was in accordance to the Truman Doctrine, which sought to counteract the geopolitical expansion of the Soviet Communism in Asia. Also, Truman had a lot of internal pressure by a faction of the public opinion that considered he was "too soft" with the Communism. For that reason, for the United States was really important to face the Communist advance supported by China and The Soviet Union.
They did not want to loss the Entire Korean Peninsula to the communists and cause a domino effect causing all of asia to fall to the communist influence
