Re. above: US & USSR had agreed (at Yalta in 2/45) to post-war 'international trusteeship' for Korea but no decision had been made as to government/governing. US improvised formula - division at 38th parallel - following landing of Soviet troops in north Korea.
US had been working towards Japanese surrender that excluded Soviet role. At Potsdam (7/45), Truman, Atlee & Chiang Kai-shek issued public statement calling for Japan's unconditional surrender. Done without consulting Soviets *and* with Truman's full knowledge of successful atomic bomb testing, US wanted to undermine negotiations for surrender underway between Soviets and Japan. US possession of operation A-bomb meant that it no longer wanted/needed USSR to enter Asian war or participate in Japan's surrender.
Truman decided to drop atomic bombs on Hiroshima (8/6/45) & Nagasaki (8/9/45) despite fact that he had received communiques from both Japanese and Soviets about imminent surrender. By doing so, US wished to limit Soviet role in Japanese surrender *and* serve notice to Soviets who already had forces in north of Korea.
Korean people learned of Japan's 8/14/45 surrender a day later on 8/15/45, same day that Truman announced partition plan. How quickly their joy and relief must have turned into disappointment and anger. Michael Hoover