Originally the fate of the Korean peninsula was to be decided by a free vote of Koreans in the immediate aftermath of the end of WWII and Japanese occupation. However, events of the Cold War quickly distorted that possibliity.
We could identify three main political considerations of the Korean War. One was that the USA was willing to intervene on the side of South Korea despite their weak position. In the context of the emerging Cold War they believed that to let Korea fall to communism would be to see 'a dagger pointed straight at Japan'.
The second political consideration is during the war itself. General Macarthur strongly argued that nuclear weapons should be used against China as his considerations were entirely military - winning a war. This was something President Truman was not prepared to do as he knew it could widen the conflict, and eventually led to Truman relieving Macarthur of his command.
The third political consideration is in the aftermath of the war. The USA suddenly took a far more dogmatic stance in Europe. They insisted that Germany should be allowed to enter the NATO alliance as an armed member (much to the great trepidation of France and Britain and the anger of the Soviet Union). It was a far more aggressive stance, and may be seen as a result of the frustration of paying such a high price for only partial success in Korea.