well actually what the film tells is an interesting story which some of our historians and authors have studied and debated about (the topic seems a little controversional) - the story belongs to the realm of 'what if' - the core of the thing is previous to March 1939 the Third Reich (and Hitler personally) were pretty keen to make an alliance with Poland - there was little to none aggressive rhetoric in German Nazi controlled media against Poland for a good couple of years - even before the start of the War the Nazi rulers didn't formulate demands that would cripple Poland much unlike what they demanded from Czechoslovakia in 1938 (the demands were ceding Gdańsk to the Germans while it was a German city anyway (with some Polish minority) and was of little value to Poland (it was not part of Poland - Poland had a couple of authorities over Gdańsk inluding duty and post) and exterritorial routes (a highway and a rail line) to the East Prussia through 'the Polish corridor' in Pommerania. (well there was also proposed plebiscite in the Polish corridor but Germany would have leave the port of Gdynia to Poland regardless of the plebiscite outcome with free access to the port for Poles through the inland)
Hitler was quite an admirer of Piłsudzki to the extent that after Piłsudzki's death he (1936) declared a national mourning in Germany - it was also a sign of friendly policies towards Poland (top Nazi echelons attended a mourning service in the memory of Piłsudzki)
The main goal of Hitler was a conquest of Russia which Poland as an ally would have much faciliated. Having Poland as an ally would enhance Hitlers option in that he would have been able to decide which way he wanted to turn his forces first - had he wanted to assault France first the Polish ally would be his shield against possible Soviet interference - with the help of Poland and most probably other central European countries like Hungary and Romania Hitler could have instead attack Russia first sooner than it actually happened having secured a line of defence (based on fortifications) on the French border - one can argue whether it would be more succesful but the further starting location of the invasion and at least a million more soldiers (Polish army) make the invasion look better on paper.
Hitler treated Poland as a friendly country in 1938 because the Poles anexed Zaolzie joining German crippling of Czechoslovakia. Ribbentrop even congratulated Poles for that. But the Polish policy makers were not that keen to join up with Hitler instead they were seeking reassurance from France and Great Britain. The turning point for German diplomacy came when Chamberlain issued guarantees for Poland's territorial integrity which Poland's ruling elite (foreign minister Beck) had taken as genuine - which in the hindsight were just luring Poland into stiff stance and eventually war with Germany - it would have given the western allies some valuable time - the time proved not enough for the French but enough for the British.
there is even a quote by Hitler going as that (Polish translation): "- Zrozumiałem, że jeżeli uderzę na Zachód, Polacy, wypełniając swe zobowiązania sojusznicze, zaatakują nas - mówił Hitler do swych oficerów na tajnej konferencji w Berchtesgaden 22 sierpnia 1939 roku. - Dlatego zdecydowałem się rozpocząć wojnę z Polską. "
which in English reads as this : I have understood, that if I strike west, the Poles, fulfilling their ally duties[to France], will attack us. That's why I decided to start a war with Poland" ( these words were allegedly spoken on a secret conferrence Hitler held with his high command in Berchtesgaden on 22nd august of 1939)
(the source is a polish historian Bogusław Wołoszański)
similar alternative history is also a part of Harry Turtledove political fiction 'The War That Came Early'
you can also choose to read this : comicism.tripod.com/390822.html - some excerpts of the Berchtesgaden conference in English
was the ultimatum to Lithuanians made in 1938 discussed there too
well the ultimatum was not much malevolent actually
if you can read Polish Nathan you can have a read of this : prawica.net/node/15900
