One additional possible strategy: The Poles might have been better off leaving forces in Polish cities that the Germans were going to isolate, rather than trying to get them out. The German offensive moved too fast for the Poles to get their people out with heavy weapons and enough organization to fight on anything like equal terms, so several times the Poles pulled divisions out of strong defenses around cities, only to have them disintegrate in the retreat. A strategy of stockpiling food and weapons in urban areas and designating troops to stay behind would have at least forced the Germans to leave behind troops to contain the Poles, probably more troops than the Poles left behind because the Germans would have to maintain a seige around entire cities. The Poles probably couldn't pull off a resistance like the Ukrainians did at Mariupol, but if they could eventually force the Germans into city fighting, they could make the fight a lot more even. Of course that would mean a lot of Polish cities destroyed, but given what the Germans did to Polish civilians, that might have been a trade-off worth making.