Why did the united states involve itself in a civil dispute on the korean peninsula in the 1950s?

The US bombed Korea and left 20% of the population dead. During the U.S. army's occupation of South Korea, relations between the U.S. and North Korea were conducted through the Soviet military government in the North. Because of North Korea's "submission" to Soviet pressures, and because of mass opposition to the U.S. occupation of the mortal enemy Japan, North Koreans in this period denounced the United States and began to form a negative view of the U.S. However, several American ministers and missionaries remained active in this period, reminding Koreans, before they were uprooted by the communist regime, that American individuals could be very helpful to the cause of Korean independence.