Although it's not that well-written (mostly due to the author not being a native English speaker), I found an interesting blog article by someone who goes by the online name giant_cheng (aka the profanely titled "Old Man and the Shit陈男旧屎"). It was compelling in its argument, for which he provides excellent evidence.
To wit, the USSR in 1950 wanted the US to intervene in Korea. The idea was to bog down both the US and China in the Peninsula. In this, it succeeded masterfully.
If you're interested in 20th century Korean history, I highly recommend it: link.
[UPDATE 2017-12-08:] My friend David in Germany, a former college roommate who sometimes reads this blog, notes that the Filippov letter that giant_cheng quotes is referenced on this page at the Wilson Center. This is a much more professional take on the same question.
[daily log: walking, 7km]