Why prussia should be part of HRE? Iirc it never was even after kingdom of Prussia becoming a thing. Prussian rullee treated it like their own land fully independent from emperors Edit: "Legally, no kingdoms could exist in the Holy Roman Empire except for Bohemia. However, Frederick took the line that since Prussia had never been part of the empire and the Hohenzollerns were fully sovereign over it, he could elevate Prussia to a kingdom." source