Well, if Japan had attacked Russia from the East, Moscow would have fallen for sure, as all those re-enforcements that came from the West to save Moscow, would have been still thousands of miles away dealing with Japan. I don't believe Japan would have had to commit a lot of troops for such a campaign, just enough to force Russia to keep those strategic reserves away from Moscow. Moscow and Lenningrad could then have been taken, and Russia would then have been way to busy trying to save herself, to worry about meeting Japan in force. Japan would then have been free to keep most of those invading forces free for Pacific and SE Asian duty IMHO!
Troy