The German aristocracy idolized the Teutonic Knights for their campaigns against Eastern Europe. They used this heritage to justify their claims that Germany was a civilized nation and Eastern Europe was full of barbaric Slavs. Germany, both the Kaiserreich and the Third Reich, traced it's military heritage back to virtues ostensibly created by the Teutonic Knights. Even in the interwar period, during the ostensibly peaceful Weimar Republic, German nationalists drew on this history to justify a revanchist war against Poland.
Prussia came about as a result of the Northern Crusades in the 13th Century, which was the settlement of Eastern Europe by Germans, courtesy of the Teutonic Knights.
While some of the German aristocracy opposed the war before it started, after it started, they were in no hurry to cede back territory they thought was rightfully German, particularly Western Poland. After Hitler won his streak of victories in 1939-40, they fell in line. After he began losing in 1943, they became restless again. To them, Hitler was a Bohemian Corporal who was treating German military tradition like a joke. He was not a man of high noble birth, of Prussian descent, he was a uncivilized brigand from Austria. Some of the aristocrats, especially Stauffenberg, had no objections to putting a boot on the backs of the Poles. If they were still winning the war, they would never have raised a hand to Hitler.
If the Allies allowed the Germans to keep Polish territory as part of some kind of peace, then that'd send a message saying that wars of aggression are justified. I don't think it'd be too crazy that the Germans, later down the line, would've used that Teutonic heritage to justify something else that couldn't be justified otherwise.