Had the allies not declared war on germany over poland, they still would have continued preparation to do so later, while germany would have prepared for an attack on the soviets. If the later conflict starts first, there is a chance, the former never does, as long as the later does not seem to go too well for germany or the soviets. If they werent going to enter over a guranteed poland, they probably wouldnt over a communist complice of the culprit. Stalin would have probably been screwed. Would be interesting to know, how much he belived in an allied declaration on germany over poland when the MR-pact was signed - cause if he wasnt sure about that, he was quite a gambler...