Why do people think that WWII would've been prevented if Hitler was stopped from becoming Supreme Chancellor?

I don't think Japan and China would fight together to fight against anyone. Although Mussolini was a dictator I don't believe he was too militaristic, at least towards his neighbors to the north. That being said, the big deal about Hitler and WW II was the final solution thing - without that it would not have been the big deal it turned out to be.