Study the history. It would have been a bloodbath. The Japanese were preparing for an attack upon the "mainland" (with many more divisions) and were resolved to fight to the death for their Emperor. I'm not so sure the Americans (Truman, the public, et al) would have been able to stomach the cost in American lives every inch would have cost them. If Guadalcanal, Peleliu, Iwo Jima, Bataan were brutal, an attack on the "mainland" would have been unimaginable. As I'm sure many of the youth here were not aware of the horrificness of the Pacific Theatre until the recent The Pacific series (and to be honest, there is no way the series could do the reality justice) if you've never read up on Bataan you should.
I'm not saying that the Japanese were not defeatable. They (the Japanese) had pretty well resolved themselves to fact of their imminent defeat. However, the Emperor was not willing to surrender unconditionally and felt that through attrition they could negotiate a surrender whereby the Emperor remained. They chose to ignore the Postdam Declaration where they were warned that failure to surrender would result in their "prompt and utter destruction".
As per the game scenario outlined, something from the Chinese or Russians would make more sense. Perhaps a follow up with American troops.