For centuries before 1945, Korea had been a unified political entity. The origins of the modern division of Korea trace to the period of Japan's colonial rule over Korea (1910-1945). During World War II, the Allied leaders fighting Japan considered the question of Korea's future after Japan's surrender in the war. The leaders reached an understanding that Korea would be liberated from Japan but would be placed under an international trusteeship until the Koreans would be deemed ready for self-rule. Beyond this rather vague agreement, much about the future of Korea was left uncertain.
Towards the end of World War II, the U.S. proposed dividing the Korean peninsula into two occupation zones (a U.S. and Soviet one). Dean Rusk and Charles H. Bonesteel III suggested the 38th parallel as the dividing line, as it placed Seoul under U.S. control. To the surprise of Rusk and Bonesteel, the Soviets accepted their proposal and agreed to divide Korea.
An assumption behind this division of Korea into two occupation zones was that this was only a temporary arrangement until the wartime agreement on the Korean trusteeship could be implemented, leading to the establishment of a unified Korean state over the entire peninsula. Subsequently, the Moscow Conference of Foreign Ministers in December 1945 resulted in an agreement on a Korean trusteeship lasting up to five years. However, with the onset of the Cold War and other factors both international and domestic, including Korean opposition to the trusteeship, negotiations between the United States and the Soviet Union over the next two years regarding the implementation of the trusteeship failed, thus effectively nullifying the only agreed-upon framework for the re-establishment of an independent and unified Korean state.:45–154
With this, the Korean question was referred to the United Nations. In 1948, after the UN failed to produce an outcome acceptable to the Soviet Union, UN-supervised elections were held in the US-occupied south only. The American-backed Syngman Rhee won the election, while Kim Il-sung consolidated his position as the leader of Soviet-occupied northern Korea. This led to the establishment of the Republic of Korea in South Korea on 15 August 1948, promptly followed by the establishment of the Democratic People's Republic of Korea in North Korea on 9 September 1948. The United States supported the South, the Soviet Union supported the North, and each government claimed sovereignty over the whole Korean peninsula.
In 1950, after years of mutual hostilities, North Korea invaded South Korea in an attempt to re-unify the peninsula under its communist rule. The subsequent Korean War, which lasted from 1950 to 1953, ended with a stalemate and has left the two Koreas separated by the Korean Demilitarized Zone (DMZ) up to the present day.