The Division 1
Planning for the occupation of Korea developed after the dropping of the atomic bomb on Nagasaki on 9 August . James Dunn, an official in the State Department who acted as chairman of the State-War-Navy 2 Coordinating Committee, informed Brigadier-General George Lincoln of the Operations Division that it was desirable that as much of Korea as possible be occupied. …[He] glanced at a map in his office and decided within ten seconds that the 39th parallel was the correct demarcation line between the zones of the American and Soviet occupation. Lincoln referred the proposal to Colonels Charles Bonesteel and Dean Rusk 3 for consideration. They reflected as to whether division at the top of the Korean peninsula might not be preferable but believed that the 38th parallel was the correct demarcation line could be acceptable if the Russians insisted on moving into the northern part of the peninsula. … The Joint War Plans Committee … thought that the 40th parallel would be a suitable line of division, since Dairen and Port Arthur would then come into the American sphere Sandusky has argued that United States troops could have assumed control of the greater part of Korea had Korea been identified as a priority and had MacArthur been willing to modify his decision to concentrate American forces in Japan prior to the official surrender on 2 September  at the ceremony he had planned.4 …
The United States aimed to prevent the Russians from occupying the whole of Korea through reaching a diplomatic agreement with Moscow to restrict Soviet occupation to the northern part of the Peninsula. Colonel Bonesteel believed the 38th parallel was the most satisfactory line in dividing Soviet and American spheres that could be arrived at; this would place Seoul, the Korean capital, in the American sphere. … The 38th parallel was not an ideal boundary and in economic terms was absurd; the 39th parallel would have been preferable. However, in favour of the 38th parallel was the fact that it appeared a rough and ready division, which might satisfy the Russians and give Seoul to the south.
The Soviet Union accepted the 38th parallel. Stalin was satisfied with this agreement and was not tempted to violate it. 5 The armistice on the 27th of July 1953 established the current Demilitarized Border.
1 Peter Lowe, The Origins of the Korean War, 2nd ed., London, Longman, 1997.[permission pending].
2 In 1945, State, War, and Navy were cabinet-level departments. Reorganization of the Department of Defense, to include Army, Navy, and Air Force, occurred in 1947.
3 Dean Rusk was later appointed to a post in the War Department in 1946 by President Truman and to Secretary of State in 1961 by President Kennedy.
4 Michael Sandusky, America’s Parallel.
5 Lowe footnoted portions of the above selection from Sandusky, America’s Parallel; Soon Sung Cho, Korea in World Politics 1940-1950, Berkeley, 1967; and Paul, ‘Diplomacy Delayed’, in Cummings (ed.), Child of Conflict.