Ducal Prussia was between (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 for the Duchy of Prussia (1525–1701), emphasizing that two Prussias existed beside each other: Royal Prussia and Ducal Prussia.Royal Prussia was held by the king of Poland, who also was the feudal lord of Ducal 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 the act called Prussian Tribut. 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.
After Germany was defeated in World War II and sanctioned by the Potsdam Conference, the southern two thirds of East Prussia was annexed by Poland, the northern part by the Soviet Union. Most of its ethnic German population had then already fled westwards in the last winter of the war, the remaining population was expulsed after the war and Polish and Russian population settled in the now Polish and Soviet territory.
Detailed Map of Prussia from about 1635 containing Ducal and Royal Prussia (in German) (http://wwwtest.library.ucla.edu/libraries/mgi/maps/blaeu/prvssia.jpg)
Duke in Gelderland, of Magdeburg, Jülich, Kleve, Berg, Stettin, Pomerania, the Kashubes, the Wends, Mecklenburg, Krossen;
Duke of Saxony, Angaria and Wesphalia, in Gelderland, of Magdeburg, Kleve, Jülich, Berg, Stettin, Pomerania, the Kashubes and the Wends, of Mecklenburg, Krossen;
Duke of Saxony, Wesphalia and Angaria, Pomerania, Lüneburg, Holstein and Schleswig, Magdeburg, Bremen, Gelderland, Kleve, Jülich and Berg, the Wends and the Kashubes, Krossen, Lauenburg, Mecklenburg;
East Prussia (German: Ostpreußen; Lithuanian: Rytų Prūsija; Polish: Prusy Wschodnie; Russian: Восточная Пруссия — Vostochnaya Prussiya; Dutch: Oost-Pruisen; Spanish: Prusia Oriental;) was a province of the Kingdom of Prussia and of the German Empire, situated on the territory of former Ducal Prussia.
East Prussia enclosed the bulk of the ancient ancestral lands of the Baltic Prussians.
The remaining German population of East Prussia was brutally oppressed and expelled by the Communist regimes.