Ducal Prussia - Wikipedia

Ducal Prussia was 1525-1657 a fief of Poland, created as a result of war (1520-1525) between Poland and the Teutonic Order. Ducal Prussia is a synonym to the Duchy of Prussia (1525-1701), emphasizing that two Prussias existed beside eachother: Royal Prussia and Ducal Prussia. Royal Prussia was held by the king of Poland, who also was the feodal lord of Ducal Prussia.

Royal Prussia and Ducal Prussia corresponds roughly to what later (1772) became West Prussia and East Prussia.

During the Reformation endemic religious upheavals and wars occurred, and in 1525, the last Grand Master of the Teutonic Knights, Albert of Prussia, a member of a cadet branch of the house of Hohenzollern, resigned his position, became a Protestant and received the title "Duke of Prussia" from the Polish king Sigismundus I the Elder. In a deal partially brokered by Martin Luther (under imperial ban since 1521), Ducal Prussia became the first Protestant state, along the lines of the later religious Peace of Augsburg[?]. When the duke Albert of Prussia died in 1569, his son Albert Frederick and then Joachim II Hector inherited Prussia. The duchy of Prussia thus came to the senior Hohenzollern branch, the ruling Margraves of Brandenburg.

The second Treaty of Thorn had left eastern Prussia as a fief of the Polish crown. In 1660, after the Second Northern War[?] between Sweden, Poland and Brandenburg, the Treaty of Welawa (Wehlau) granted full sovereignty to Frederick William I, the "Great Elector", of the Brandenburg Hohenzollerns as "Duke of Prussia". Thereby Ducal Prussia loses it's status of polish fief and becomes a part of Brandenburg-Prussia.

The treaty also prescribed that when the Hohenzollern rule in the Ducal Prussia expired, the land would return to Poland as its integral part. (Hohenzollern rule expired only in 1919, when Wilhelm II of Germany abdicated as German Emperor and King of Prussia, but East Prussia didn't return to Poland until 1945, and even then only its southern part.)

Also see :

List of Kings of Prussia