A question I can answer! Well, answer briefly. Most of what I know about this topic comes from Manchester's Churchill biography. Basically, Roosevelt had a life-long fascination with China--some of his relatives had lived there and worked there in the past and he had some very romantic ideas about it, and so when he joined the war effort he constantly pushed a (reluctant) WSC and Stalin to treat China as an equal partner in the alliance. This is part of what led Chiang Kai Shek to be welcomed at the Cairo conference of 1943 which dealt with the topic of the Pacific settlement. It was one of his constant sticking points about the post war settlement. So the initial impetus for it was FDR though obviously it was finally carried out by Truman.