Imajin said:
Prussia was named after the Prussians, a pagan people who were conquered, forcibly Christianized and later Germanized by the Teutonic Order.
What if, around the year 1000, one of the tribal Prussian leaders becomes a Christian, and unites the Prussian people, forming his own Kingdom of Prussia a few hundred years earlier? Does this POD make sense?
Definitely, it makes. But I think that sequence should be turned: first, The Prussian Leader should unite Prussian people, and
then become Christian.
And that would change a lot. Instead of wild tribes, there would be state, legitimized by whole Europe. So, no Teutonic Order in this part of Europe. Instead, Prussia (or whatever would be the name of this country) could become an important player in this part of Europe. Its relations with Poland would be "usual": border conflict, with raids, taking prisoners, times of "everlasting peace" lasting for few years at most.
More interesting would be Prussian-Lithuanian relations. Possibilities vary from some kind of commonwealth (as both those nations would be ethnicaly similiar), toward expanding Prussia east, or Lithuania west.
And without shadow of Teutonic Order, Lithuania could became in XIII/XIV century became an important player to take advantage of fall of Mongolian empire in Russia.