How are you gonna do that, WWII started because of Poland, perhaps if the Soviets attack Poland instead of dividing it with Germany, but Stalin was for to careful for that.
The most likely senario I can think of is that the UK and France act much like OTL Spain become(, perhaps becoming facist in the 30's), because of the lack of support and fear of a Soviet invasion they give the Nazi's some land in return for a militairy support.
I don't think Stalin would have attacked, but let's just say he underestimes the power of the German militairy and attacks. Voila
The thing is America wouldn't invent nuclear weapons, why would they, I'll place my bet on the Germans.