I see it this way:

Hitler throws the dice anyway. He invades Poland. Hitler would have assumed the position that his opponets would not want war and he could bluff his way to another victory.
Britain and France declare war but neither is in any position to launch an offensive; France is unwilling and Britain lacks the military base to go it alone.
Stalin while he does not like the prospect of Germany taking Poland is in the same boat. Declaring war right off as Germany rolls easily over Poland is a big risk. This is particularly true given that he has just had a rough go with the Japanese and Finns and his army is in a state of transition with the purges, reorgainzation, and what-not.
This leaves Germany faced with a single front and a second possible one that is not currently active (the Soviets). The question is do the Soviets start a war unilaterally with Germany or adopt a safer wait and see attitude while strengthening their Stalin line on their pre-war border? If the Soviets do go to war and Germany crushes their initial offensives the danger for the Soviets is that Germany is now about 300 miles closer to Moscow right from the start and the Soviet army is out of positon, off balance, and getting hacked to pieces one more time in a botched offensive.