It sounds like this became a three-way war: USSR vs US & UK vs Japan. I assume that the US and UK were still at war with Japan in the Pacific?
I played a game a while back that was kind of similar. Basically, Japan never went to war with USA or UK but just attacked China and Russia. On the other side, Germany and Italy never attacked Russia and only fought with USA and UK. Well, Germany and Italy ended up getting pounded into the dust. Japan actually made good progress capturing all of China and getting close to taking Moscow. One reason for this is most of Russia’s heavy stuff was up against the German border and it took time to get it all east to face the Japanese. Once Japan got through that 18 infantry, there wasn’t much to stop them from making quick progress. Once US and UK got finished with Germany and Italy, they decided to side with Japan and declared war on Russia to rid the world of Communism. Once that happened, Russia fell pretty quickly. The large US and UK forces that were already in Europe just leap-frogged each other taking Soviet territories and closing in on Moscow from the west while Japan closed in from the east with their massive air force.
One strange thing was here Japan had this huge and powerful navy which was basically useless. All those ships ended up clustered in SZ 6 and just rusted away from inaction.