Polish forces under Grand Crown Hetman Mikołaj Firlej gathered near Koło and in January struck towards Pomesania towards Königsberg, laying siege to Marienwerder (now Kwidzyn) and Preußisch Holland (now Pasłęk). The siege was slow, however, since the Polish forces lacked artillery power. The Polish fleet began a blockade of Teutonic ports. The Knights, in the meantime, took the Warmian city of Braunsberg (now Braniewo). The 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 the Duchy of Masovia and Gdańsk striking the nearby Teutonic fortifications. Teutonic forces were on defense, waiting for reinforcements from Germany, which arrived in the summer of 1520. 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 Greater Poland, taking Międzyrzecz. The Germans took Wałcz, Chojnice, Starogard Gdański, and Tczew and started a siege of Gdańsk, but they retreated when faced with Polish reinforcements and plagued by financial troubles (German reinforcements, mostly mercenaries, refused to fight until paid). Polish forces retook Tczew, Starogard, and Chojnice. The Teutonic Knights retreated towards Oliwa and Puck, pursued by Polish forces. The Polish side was then struck with 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 approaching Płock and Olsztyn. Olsztyn was successfully defended by the Poles under the command of Nicolaus Copernicus.

At that point, 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 5 April 1521 in the Compromise of Thorn.