The facts are that the USSR and the Korean people (many of them being led by Kim Il Sung) were the ones who drove Japan out of Korea. The US didn't want the USSR to have influence in all of Korea so they drew a line at the 38th parallel. The USSR agreed to that line because they didn't see it as a hill they wanted to die on (they didn't want to chance a war with the US since they had just lost 27 million people in WWII).