Polish–Teutonic War (1519–21)

Polish–Teutonic War of 1519–1521 (Reiterkrieg, horsemen's war, Wojna pruska, Prussian War) was the war between the Kingdom of Poland and the Teutonic Knights, fought from 1519 to 1521. The war ended with the armistice in 1521. Four years later, with the Treaty of Kraków, part of the Catholic Monastic state of the Teutonic Knights became secularized as the Duchy of Prussia. The reigning Grand Master Albert of Hohenzollern-Brandenburg-Ansbach became the first Duke of Prussia by paying the Prussian Homage as vassal to his uncle, Polish king Sigismund I the Old.

Prelude
In the late 1490s, the Order, which since the Second Peace of Thorn (1466) had its state under Polish suzerainty, developed the idea of electing only an Imperial Prince as future Grand Master, who as subject to the Emperor could resist having to pay homage to Kings of Poland. The order itself, which was present not only in Prussia but throughout the Empire, was also subordinate to the Holy Roman Emperor who, since 1501, had objected to Duke Frederick of Saxony, Grand Master of the Teutonic Order and Reichsfürst, offering tribute to the Polish king.

When Frederick died in December 1510, Albert of Hohenzollern was chosen as his successor early in 1511 in the hope that his relationship to his maternal uncle, Sigismund I the Old, Grand Duke of Lithuania and King of Poland, would facilitate a settlement of the disputes over eastern Prussia. The new Grand Master, aware of his duties to the empire and to the papacy, refused to submit to the crown of Poland. As war over the Order's existence appeared inevitable, Albert made strenuous efforts to secure allies and carried on protracted negotiations with Emperor Maximilian I.

However in the meantime, the Order had been looking for other allies. In 1512, Muscovy invaded the Grand Duchy of Lithuania, which was in personal union with Poland. The Order was supposed to help the Duchy, but it refused, angering the king of Poland and Lithuania. In 1517, the Teutonic Order signed an alliance with Vasili III of Muscovy.

Albert, now feeling he held the upper hand, demanded from the Polish king the return of Royal Prussia and Warmia territories, and a large remuneration for "Polish occupation" of those territories. In response, first the diet of Prussia (Landtag or sejmik), and then, in December 1519, the main Polish sejm, declared that a state of war exists between Polish Kingdom and the Order. Lithuania, however, refused to aid Poland in this war, being occupied with the Muscovy threat.

The war
Polish forces under Grand Crown Hetman Mikołaj Firlej gathered near Koło and in January struck towards Pomesania towards Königsberg, laying siege to Marienwerder (Kwidzyn) and Preußisch Holland (Pasłęk), but the siege was slow since the Polish forces lacked artillery power. The Polish fleet started a blockade of Teutonic ports. The Knights, in the meantime, took the Warmian city of Braunsberg (Braniewo). Polish army received artillery reinforcements in April and took Marienwerder and Preußisch Holland that month, but failed to retake Braunsberg.

The war grew, with Polish forces from Duchy of Masovia and Gdańsk striking the nearby Teutonic fortifications. In the meantime, Teutonic forces were on defense, waiting for reinforcements from Germany. Those reinforcements arrived in summer of 1520, and in July, the Teutonic army started an offensive, attacking Masovia, Warmia and Łomża territories, laying siege to Lidzbark Warmiński. In August another group of German reinforcements attacked Wielkopolska, taking Międzyrzecz. Germans took Wałcz, Chojnice, Starogard, Tczew and started a siege of Gdańsk, but retreated faced with Polish reinforcements and plagued by financial troubles (German reinforcements, mostly mercenaries, refused to fight until paid). Polish forces retook Tczew, Stargard and Chojnice. The Teutonic Knights retreated towards Oliwa and Puck, but were pursued by Polish forces. This time however the Polish side was plagued by financial troubles, and the "pospolite ruszenie" forces were also tired. The Teutonic Knights seized their chance and launched a counteroffensive, taking Nowe Miasto Lubawskie and approacing Płock and Olsztyn. Olsztyn was successfully defended by the Poles under the command of Nicolaus Copernicus.

At that time, however, the Ottoman Empire invaded Hungary, and the new Holy Roman Emperor, Charles V, demanded that the Teutonic Knights and Poles stop their hostilities and aid the defense of Europe against the infidels. Both sides, tired with the war, agreed to an armistice on April 5, 1521, in the Compromise of Toruń.

Aftermath
During the four-year truce, the dispute was referred to Emperor Charles V and other princes, but no settlement was reached, and Albert continued his efforts to obtain help in view of the inevitable end of the truce.

Eventually, in Wittenberg, Albert met and was advised by Martin Luther to abandon the rules of his Order, to marry, and to convert Prussia into a hereditary duchy for himself. Albert agreed, converted to Lutheranism in 1525, and resigned from the Hochmeister office to assume the Prussian Homage from his uncle Sigismund I the Old, King of Poland, the hereditary rights to the now-secularized Duchy of Prussia as a vassal of the Polish Crown. The Prussian Landtag diet assembled in Königsberg, where all "Stände", led by the influential Bishop of Samland Georg von Polenz, embraced both the new Duke and Protestant Reformation to Lutheran faith. Thus, the Order was ousted after facing a century of opposition by the Prussian Confederation. The Order elected a new Grand Master who tried to fight the loss of power in the Prussian territories by political means, but could never regain any influence there.