Prussia
Northeast Germany was Protestant, so Prussians were mostly Protestant. But there were a lot of Catholic people in the Rhineland, East Prussia, Posen, Silesia, West Prussia, and Ermland. The states of south Germany (especially Austria and Bavaria) were Catholic, so they did not want Prussians to rule them. Prussia was mostly German, but in the late 18th century, the new Polish areas had a lot of Polish people too. In 1918, these Polish areas were given to Poland, and in 1945, Pomerania and East Prussia were given to Poland. Northern East Prussia, specifically Kaliningrad, was given to Russia.