It was never part of Poland, unless you're going waaaay back to the Teutonic knights. Even then, it was controlled by an ethnically-German ruling dynasty with i

It was never part of Poland, unless you're going waaaay back to the Teutonic knights. Even then, it was controlled by an ethnically-German ruling dynasty with its power base outside of the Polish-Lithuanian Commonwealth, similar to Normandy in late medieval France. In the aftermath of WWII, Germans in East Prussia were expelled, and the territory was split 50/50 between the USSR and Poland. No state other than Germany has a pre-WW2 claim, and they formally renounced that claim in the final peace treaty around 1990.