At the Tehran Conference in November 1943 and the Yalta Conference in February 1945 , the Soviet Union promised to join its allies in the Pacific War within three months of victory in Europe . On August 8 , 1945 , after three months to the day , 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 , the US government decided to propose the 38th parallel as the dividing line between a Soviet occupation zone in the north and a US occupation zone in the south . The parallel was chosen as it would place the capital Seoul under American control . 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 . The division placed sixteen million Koreans in the American zone and nine million in the Soviet zone . Soviet forces began amphibious landings in Korea by August 14 and rapidly took over the northeast , and on August 16 they landed at Wonsan . On August 24 , the Red Army reached Pyongyang . US forces did not arrive in the south until September 8
Yalta Conference
Yalta Conference
History of North Korea