<P> At the Tehran Conference and the Yalta Conference , the Soviet Union promised to join its allies in the Pacific War in two to three months after victory in Europe . On August 8 , 1945 , three months to the day after the end of hostilities in Europe , and two days after the atomic bomb was dropped on Hiroshima , the Soviet Union declared war on Japan . Soviet troops advanced rapidly , and the US government became anxious that they would occupy the whole of Korea . On August 10 , 1945 two young officers -- Dean Rusk and Charles Bonesteel -- were assigned to define an American occupation zone . Working on extremely short notice and completely unprepared , they used a National Geographic map to decide on the 38th parallel . They chose it because it divided the country approximately in half but would place the capital Seoul under American control . No experts on Korea were consulted . The two men were unaware that forty years before , Japan and pre-revolutionary Russia had discussed sharing Korea along the same parallel . Rusk later said that had he known , he `` almost surely '' would have chosen a different line . The division placed sixteen million Koreans in the American zone and nine million in the Soviet zone . To the surprise of the Americans , the Soviet Union immediately accepted the division . The agreement was incorporated into General Order No. 1 ( approved on 17 August 1945 ) for the surrender of Japan . </P>
when did korea separate into north and south
17 August 1945