I disagree with Cook concerning a probability of Polish DoW on Germany. A war with Germany (or enybody with possible exception of Lithuania) was the last thing Poland wanted. Polish Army had 40 infantry divisions (including 10 reserve divisions, weaker than regular ones), 11 cavalry brigades, 2 motorized brigades (including 1 still being organized), 3 mountain brigades, relatively weak artillery and weak air force. Polish commanders knew very well Polish Army had no chance at all of defeating the Wehrmacht. However, they believed they might have been able to resist until western allies attack, as it had been promised to the Poles. They knew they could not stand for long in a defensive war, so an aggressive strategy was even less attracticve.
First, mobilization In fact, on September the 1st 1939 Poles had circa 65-70% of their forces mobilized - mostly because western allies pressed on Warsaw to cancel full mobilization started at the end of August fearing that it would provoke the Germans. Allegedly French ambassador even threatened with cancelling any French support for Poland. If Paris and Londodn reacted so in case of defensive mobilization, what would they do in case of Polish aggression?
Two, Poland had no supplies to fight longer than 2-3 months (and that is somewhat optimistic). Polish commanders hoped for western supplies arriving via Romania.
Three, as I mentuioned Polish army was much, much weaker than German Army and Poles knew that.
Four, AFAIK, Poland had no serious plans of any offensive against Germany (not after 1937 for sure); for intervention in Danzig, sure. Bullying Lithuania? Easily. But you don't improvise an attack against so powerful foe Germany was. Even Polish commanders at the time were not so reckless.
Five, it doesn't make sense for Poland to attack Germany because Hitler was about to become buddy with Stalin. Such an alliance would have been made even more attractive to Hitler in case of Polish attack: Germany could use an ally and are willing to pay with eastern Poland; USSR has easier job with Polish Army fully engaged against Germany; neither Moscow, nor Berlin have any warm feelings towards Warsaw. In short, the moment Hitler and Stalin decided to shake hands, Poland was screwed, and Poles knew that.
Six, Polish chances to reach Berlin are small, and capturing it, even smaller. Poles need to leave at least 20 divisions at home, or Germans or Soviets will ride straight into Warsaw; some forces need to cover flanks (Polish Army is rather slow) and secure supply lines; IMHO that would make Polish attack force at best 10 infantry divisions, 6 cavalry and 1 motorized brigade. That force has to cross the Oder (which is not so easy) and assault a big city full of people, with not only the Werhmacht defending it, but also SS, SA, etc. And
Seven, assuming by some miracle Poles capture Berlin - so what? Ther forces are decimated, German HQ and government were evacuated to Hamburd or some other place. The war goes on and Poland has no strength to continue.