The United States entered the Korean War in June of 1950 as part of a United Nations effort to assist South Korea in its civil war with North Korea, which was supported by both China and the Soviet Union. The origins of the conflict began in 1945, when Korea was removed from Japanese rule at the end of World War II, and it was during this conflict that the division along the 38th parallel was established to separate North and South Korea.
The United States and the Soviet Union, who had been allies during World War II, took control of South and North Korea respectively, setting the stage for war. The division between the two countries was meant to be temporary, but the Soviet Union supported the installation of a dictator, Kim Il Sung, and the formation of the Democratic People’s Republic of Korea as an alternative to a united, single Korea. Although the DPRK was established in North Korea under communist rule and the Republic of Korea to the South was subject to American-style democratic rule, each respective nation's leader wished to unite the divided nation under a single flag. It is the dramatically different political ideologies that led to the civil war.