Hitler gave orders that if the French so much as fired a shot when the Germans marched into the Rhineland, that his troops were to turn back. It never happened, and from the point on, Hitler knew Britain and France wouldn’t lift a finger to stop him.
Our present war is a result of Bush's having to decide between the Chamberlain and Churchill approach to the problem -- and I think history will conclude that he chose right.
Had France acted against Germany, one supposes the situation would have been at least as messy as the one in Iraq today; and it's difficult to claim that Germany wouldn't have tried to re-militarize in some other way. (After all, they had been secretly, if slowly, doing so since 1919....)
Hitler was so nervous he couldn’t sleep the night before.