Let's say in the late 1930s the Western powers decided to take a neutral stance in the incoming Nazi-Soviet and Sino-Japanese conflicts (thus sacrificing Poland and China in the process) in the expectation that they will destroy each other eventually, and let's assume again that the Nazis and the Japanese promised (by making treaties, or whatever) that they won't attack anyone else other than Soviet Union and China, respectively.
So in this scenario, where Nazi Germany and Soviet Union, as well as Imperial Japan and China, unleashed their full military might against each other, without any helps or other interferences from outside powers, who would ultimately win? And how would the end result look like?