Japan had ruled the Korean peninsula between 1910 and 1945. When Japan surrendered in August 15, 1945, the 38th parallel was established as the boundary between Soviet and American occupation zones. This parallel divided the Korean peninsula roughly in the middle. In 1948, this parallel became the boundary between the Democratic People's Republic of Korea (North Korea) and the Republic of Korea (South Korea), both of which claim to be the government of the whole of Korea.

Explaining the choice of the 38th Parallel, US Colonels Dean Rusk observed, "even though it was further north than could be realistically reached by US 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 US 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 Soviet government would agree to this. Soviet leader Joseph Stalin, however, maintained his wartime policy of co-operation, and on 16 August the Red Army halted at the 38th Parallel for three weeks to await the arrival of US forces in the south.

On 7 September 1945, General Douglas MacArthur issued Proclamation No. 1 to the people of Korea, announcing U.S. military control over Korea south of the 38th parallel and establishing English as the official language during military control. MacArthur ended up being in charge of southern Korea from 1945 to 1948 due to the lack of clear orders or initiative from Washington, D.C.