It's difficult to know which side is lying about their claims around 1939 (although 'our' side doesn't even tell us what the german argument is save for a radio station bombing so that should be a big hint).
However, admitted history in 1921 demonstrates a preview of the exact claims Hitler would later make, that Polish militias were slaughtering germans en masse in what should have been german land as was democratically voted on.
https://youtu.be/HwDAorQnd4U?t=14
Grospoliner -1 points 2.0 years ago
Every argument about Germany's invasion of Poland ignores pre world war 1 history. No, the territories of Old Prussia were not Rightfully part of Brandenburg Prussian domain, they were an entirely separate nation formed by crusaders under papal approval after it was decided to christianize the baltic pagans in modern day latvia and estonia. None of the region was ever formally part of the Empire, and legitimate claims in the region after Brandenburg broke from Polish rule would have died with the HRE. By that point Poland had ruled it for close to three hundred years, after defeating the crusader state that was controlling the regiin, and only stopped doing so after being hit with multiple coalition wars against them by Austria Germany and Russia in the 18th century. Poland existed long before the formation of the German Empire, so no the land was not simply German land.