In OTL FDR in January 1943 (Casablanca Conference) had the plans of 'Unconditional Surrender' to Germany and Japan. Nothing else was allowed.
Germany was going to be turned an agricultural state by the Morgenthau plan directives. It was called (notably by Liddel Hart) WW2's worst blunder.
This stiffened German resistance. Now the German public (largely due to Goebbels) knew that the Allies were not only prepared to defeat Hitler, but also to destroy Germany. They didn't have a choice other than to swear an oath to Hitler and fight for their country.
So, WI FDR wasn't so stiff with his 'Unconditional Surrender' slogan? WI they informed that they only wanted Hitler out of the way, like in Woodrow Wilson's 1918 'Fourteen Points'? Would Hitler be assasinated, now that the Germans knew all the Allies wanted was Hitler out of the way?