The Polish–Teutonic War of 1519–1521 CE was a battle fought between Teutonic Knights and the Kingdom of Poland, ending with a cease-fire in April 1521 CE. Four years later, under the Treaty of Kraków, part of the Catholic Monastic nature of the Teutonic Knights became secularized as the Duchy of Prussia.
After the Second Peace of Thorn (1466 CE), the Teutonic Order was under Polish rule. In the late 1490s CE, the Order manifested the idea of electing only an Imperial Prince as ultimate Grand Master, who as subject to the Emperor could continue having to pay homage to Kings of Poland. The Order was present in Prussia but throughout the Medieval Empire. It was also subject to the Holy Roman Emperor. Since 1501 CE to Duke Frederick of Saxony, he had opposed Grand Master of the Teutonic Order and Reichsfürst, offering recognition to the Polish king.
Frederick ultimately died in December 1510 CE, and Albert of Hohenzollern was picked as his successor early in 1511 CE in the hope that his relationship to his uncle, Sigismund I the Old, Grand Duke of Lithuania and King of Poland, would promote an agreement of the disputes over eastern Prussia. The new Grand Master, conscious of his duties to the papacy and to the empire, declined to submit to the crown of Poland. As the war over the Order’s survival appeared inevitable, Albert made vigorous efforts to secure allies and carried on lengthy negotiations with the robust Emperor Maximilian I.
Polish army under Grand Crown Hetman Mikołaj Firlej assembled near Koło and in January moved towards Pomesania towards Königsberg, laying siege to Kwidzyn (Marienwerder) and Pasłęk (Preußisch Holland). The offense was slow, however, since the Polish forces didn’t have the necessary cannon power. The Polish fleet started a blockade of Teutonic ports. The Knights, in the meantime, sized the Warmian city of Braniewo (Braunsberg). The Polish unit received artillery reinforcements in April and took Preußisch Holland and Marienwerder that month but failed to retake Braniewo (Braunsberg).
The war evolved, with Polish forces from the Gdańsk and Duchy of Masovia hitting the nearby Teutonic castles. Teutonic forces were on guard, waiting for reserves from Germany, which entered the war in the summer of 1520 CE. In July, the Teutonic army launched an offensive, attacking Warmia, Masovia, and Łomża regions, laying siege to Lidzbark Warmiński. In August, another army of German reinforcements penetrated Wielkopolska, taking Międzyrzecz. The Germans took Chojnice, Wałcz, Tczew, and Starogard Gdański and launched a siege of Gdańsk. Still, they left when faced with Polish reinforcements and tormented by financial troubles (German reinforcements, primarily mercenaries, stopping fighting until duly paid). Polish army retook Starogard, Tczew, and Chojnice. The Teutonic Knights withdrew towards Puck and Oliwa, pursued by Polish forces. The Polish team was then hit with financial troubles, and the “pospolite ruszenie” forces were also exhausted.
At that time, the Ottoman Empire trespassed and carried an invasion of Hungary, and the new Holy Roman Emperor, Charles V, commanded that the Poles and Teutonic Knights stop their resentments and assist the defense of Europe against the infidels. Both sides, tired of the war, agreed to a truce on 5 April 1521 CE in the Compromise of Toruń.
During the tiring four-year truce, the dispute was transferred to Emperor Charles V and other monarchs, but they sadly reached no settlement. Albert continued his attempts to obtain help because of the impending end of the truce.
Finally, in Wittenberg, Albert met and was suggested by Martin Luther to reject his Order’s rules, marry, and convert Prussia into a hereditary duchy for himself. Albert agreed and turned to Lutheranism in 1525 CE.
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