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Korea divided (1945–1949)

At the Tehran Conference in 1943 and the Yalta Conference in February 1945, the Soviet Union promised to join its allies in the Pacific War within three months of the victory in Europe. Germany officially surrendered on 8 May 1945, and the USSR declared war on Japan and invaded Manchuria on 8 August 1945, three days after the atomic bombing of Hiroshima. By 10 August, the Red Army had begun to occupy the north of Korea.

On 10 August in Washington, US Colonels Dean Rusk and Charles H. Bonesteel III were assigned to divide Korea into Soviet and US occupation zones and proposed the 38th parallel as the dividing line. This was incorporated into the US General Order No. 1, which responded to the Japanese surrender on 15 August. Explaining the choice of the 38th parallel, Rusk observed, "Even though it was further north than could be realistically reached by U. S. forces in the event of Soviet disagreement... we felt it important to include the capital of Korea in the area of responsibility of American troops". He noted that he was "faced with the scarcity of U.S. forces immediately available and time and space factors which would make it difficult to reach very far north before Soviet troops could enter the area". As Rusk's comments indicate, the US doubted whether the Soviets would agree. Joseph Stalin, however, maintained his wartime policy of cooperation, and on 16 August, the Red Army halted at the 38th parallel for three weeks to await the arrival of US forces.

On 7 September 1945, General Douglas MacArthur issued Proclamation No. 1 to the people of Korea, announcing US military control over Korea south of the 38th parallel and establishing English as the official language during military control. On 8 September, US Lieutenant General John R. Hodge arrived in Incheon to accept the Japanese surrender south of the 38th parallel. Appointed as military governor, Hodge directly controlled South Korea as head of the United States Army Military Government in Korea (USAMGIK 1945–48).

In December 1945, Korea was administered by a US–Soviet Union Joint Commission, as agreed at the Moscow Conference, to grant independence after a five-year trusteeship. Waiting five years for independence was unpopular among Koreans, and riots broke out. To contain them, the USAMGIK banned strikes on 8 December and outlawed the PRK Revolutionary Government and People's Committees on 12 December. Following further civilian unrest, the USAMGIK declared martial law.

Citing the inability of the Joint Commission to make progress, the US government decided to hold an election under UN auspices to create an independent Korea. The Soviet authorities and Korean communists refused to cooperate on the grounds it would not be fair, and many South Korean politicians boycotted it. The 1948 South Korean general election was held in May. The resultant South Korean government promulgated a national political constitution on 17 July and elected Syngman Rhee as president on 20 July. The Republic of Korea (South Korea) was established on 15 August 1948.

In the Soviet-Korean Zone of Occupation, the Soviets agreed to the establishment of a communist government led by Kim Il Sung. The 1948 North Korean parliamentary elections took place in August. The Soviet Union withdrew its forces in 1948 and the US in 1949.