Korea was annexed by Japan until 1945 when the Soviet Union invaded. America, fearing that the Soviet Union sought to take over the entire peninsula, intervened from the south. At the time, US feared the spread of communism through the Soviet
Union. Thus, America felt compelled to intervene and "contain" the Communism.
Korea was separated into the north and south: the north supported by the Soviet Union and the south supported by America. On June 25th, 1950, North Korean troops attacked, and in response, the United States and the United Nations combined forces and fought back, thwarting the invasion.