This may sound ignorant, but I do not understand why Korea was under subject for the rivalries of the the US and Russia.
Japan seemed more of a likely target to divide as it was much more industrialized and richer comparably to the economically poor Korea.
Sorry if I sound silly, history isn't my forte.
After WWII, the U.S. occupied all of Japan, and forced a constitution that ensured that Japan would not have another military. After that point, Japan's economy began to flourish even more, because the U.S. was taking care of Japan's military needs. (It didn't need to spend anything on the military).
After Japan's defeat, Korea was partitioned because of rival USSR and U.S. interests. It was meant to be a temporary thing, but the Korean War solidified the separation. After WWII, The UN issued mandates of land solutions to colonial boundaries and war torn areas (such as the Arab mandates). Korea was one of them.