The Union of Potsdam - Hohenzollern Poland
Poland in 1385, Brandenburg in 1385, Expansion of 1385-1699, Royal Prussia, Ducal Prussia
In 1385, the Margrave of Brandenburg, Friedrich II von Hohenzollern, inherited the Kingdom of Poland through his marriage to Jadwiga. In return for taking the throne, Friedrich promised certain rights to nobility in a document signed in Potsdam (Polish:
Podzdąb). The document, and its signing, are known as the "Union of Potsdam" (Polish:
Unie w Podzdąb).
Banner of Poland-Brandenburg, invariably showing either the black/white of Hohenzollern or the red eagle of Brandenburg. In later years, the red eagle was dropped in favor of only the Polish white eagle.
In the decades following the union, the Hohenzollerns established themselves in the old capital of Poznań due to its closer proximity to Brandenburg, and thoroughly immersed themselves in the rites and culture of Poland. They polonized their descendants, as did the nobility, in an effort to strengthen the future state. The Teutonic Knights were also brought to heel, and future expansion took place into Pommerania (Pomorze) and Silesia (Śląsk).
Brandenburg-Poland eventually became just "Poland" again in 1699 with the Declaration of Gnieżno (Deklaracja w Gnieżno) which dissolved the Margraviate of Brandenburg and recast it as "Bamboria" (Polish:
Bamborska), henceforth a voivodeship of Poland.
Note: "Bamborska" is a corruption which emerges ITTL from the Slavic "Braniborsk" and "Branborska" (I kind of just think it sounds better is all)