And if Tansill is to be believed in Back Door To War, the Polish government had decided to acquiesce to the Germans as Czech government had done and Britain talked them out of it with promises of support.
David Hogan says the same thing in "The Forced War", and he's one of the few historians who actually bothered to learn Polish and read the Polish diplomatic communiques of the time.
Germany wanted a war with Russia, and Poland just happened to be a stepping stone that could either cooperate like Slovakia and Hungary, or resist like the Czechs and be swallowed. Both the German and Polish military establishments had delusions of grandeur stemming from the 1920's, and involving the revision of the border either towards the Elbe for the Poles or towards the Bug for the Germans (the "Hitler rearmament" was simply a dusting off of a Wehrmacht plan from the 1920's). Britain and France wanted a war with Germany to keep her down after she started to get up. Stalin of course wanted to conquer Europe. Everyone had different aims, but to get them all warring with each other and to make the war last 6 years took an incredible lack of foresight and naive belief in the superiority of their own forces and a supposed lack of consequences from the fighting on all sides. In the end, it can be said decisively, Europe lost from this stupidity, and permanently. The 60 million dead would have been 100,000,000 or more people today.