The DPRK-- or North Korea as it is known today -- had China and the U.S.S.R. as its main allies. Together they fought for dominance of the Republic of Korea -- known today as South Korea.
The following countries played a significant role in the U.N forces: Australia, Belgium, Canada, Colombia, Denmark, Ethiopia, France, Greece, India, Italy, Luxembourg, the Netherlands, New Zealand, Norway, the Philippines, South Africa, South Korea Sweden, Thailand, Turkey, the United Kingdom and the United States.
After WW II, the Soviets occupied the Democratic Peoples Republic of Korea and the United States occupied the Republic of Korea. Both were de facto states; neither one recognized the legitimacy of the other. In January 1950, the United States withdrew from the ROK, and Secretary of State Dean Acheson stated that the Korean peninsula was outside of the United States' defense perimeter. Just six months later in June, war broke out along the 38th parallel when North Korean troops headed south, attacking several strategic points along the way as they made their way toward Seoul, their destination point. The United Nations unanimously condemned this as "a breach of peace." President Harry Truman enforced what he called "a police action," committing the United States to the U.N Forces and appointing Gen. Douglas MacArthur in charge.