When Germany invaded Poland, the vast majority of its (believe it or not limited) fighting power was fully tied up in that conflict. During that same time, The larger, stronger, better equipped French army could have (with the aid of the BEF) invaded Germany from its border with it (and possibly even through Belgium) and caused absolute chaos in the German's plans for expansion. ven if the French/British had failed to make that invasion decisive, it would almost certainly have broken the back of the entire chain of lightning victories and support for Hitler that came later. At the very least, that preemptive invasion attempt would have been able to mobilize the French Army enough to completely forestall the terrible consequences of low morale and rapid military collapse that eventually befell France when it actually was invaded so spectacularly.
The above is in my view a near certainty if it had happened that way. More speculatively, had Churchill been Prime Minister instead of Chamberlain at the time of the Poland invasion (instead of becoming PM right at the time of the invasion of France), the British response would have been much stronger and that would have prompted an also stronger French response that could have dramatically curtailed the scope of the European war.
Hitler's army in 1939 simply wasn't good or well provisioned enough to sustain anything serious against any major enemy. It pulled off what it pulled off in the following months more through a mix of luck and sheer blunder by its enemies.