Because of its geographical position, pointing like a dagger at Japan, the Korean peninsula has always had great strategic importance. Its significance was hidden for 1000 years while China held sway over the peninsula, but the rise of Japan and the emergence of a Russian presence in the Far East in the nineteenth century changed the picture. Korea became the cockpit of a great-power competition that Japan eventually won. In 1910 it annexed Korea.
That two Korean states existed in 1950 was an outcome of arrangements for the surrender of Japan in August 1945 which had resulted in the entry of both Soviet and American forces to the peninsula. The ostensible purpose of this deployment was to take the surrender of Japanese troops, but both great powers were determined to maintain a foothold in this strategic area. The demarcation line on the 38th Parallel was rapidly transformed into a quasi-border as relations between the Soviet Union and its former wartime allies worsened with the onset of the Cold War, and both sides encouraged political factions sympathetic to themselves.
Partition became inevitable when negotiations to provide a unified Korean administration fell down. In 1948 the United Nations oversaw the creation of the Republic of Korea (ROK) in the south with Syngman Rhee as President, to which the Russians responded by establishing the Democratic People’s Republic of Korea (DPRK) in the north with Kim Il-sung at its head. Koreans were unreconciled to this outcome, and on both sides of the 38th Parallel there was determination to reunify the peninsula. Border incidents were common.
The June 1950 invasion was initiated by Kim Il-sung, but Josef Stalin's approval was crucial and Soviet assistance in the form of arms and advice was provided to the Korean People's Army. Catching the South Koreans by surprise, the Korean People's Army (KPA) made rapid progress, capturing the southern capital Seoul within three days of its onslaught. However, with the United States to the fore, the United Nations Security Council had called for a withdrawal, and when this was ignored called on members to assist South Korea. Early in July it set up a UN Command, responsibility for which was delegated to the United States. General Douglas MacArthur, the Supreme Commander, Allied Powers in Japan, was appointed as Commander-in-Chief, UN Forces in Korea.