This essentially amounted to a switching of sides which the German Order immediately took as a mortal threat, and a war broke out between Poland and the Teutons.

The Teutonic Order also returned Eastern Pomerania to Poland after nearly 150 years and ceded the bishopric of Warmia, which together formed the so-called Royal Prussia, as both lands fell under direct rule of the Polish King.

Tension quickly flared up afterward, and this was soon followed by the War of the Priests (1467–1479), a drawn-out dispute over the independence of Warmia, in which the Knights sought revision of the Peace.

After the victory by the Polish and Lithuanian forces at Grunwald during the Polish–Lithuanian–Teutonic War (1409–1411), the Prussian estates eagerly pledged allegiance to King Władysław II Jagiełło (Jogaila) of Poland.

In the succeeding wars the Prussian estates opposed any conflict, and pushed the Grand Masters of the Teutonic Knights to make peace.

Two days later the confederacy started its rebellion and soon almost all Prussia, except for Marienburg, Stuhm (Sztum), and Konitz (Chojnice), were free from Teutonic rule.

After negotiating the exact conditions of incorporation, the king agreed and delegates of the Prussian Confederation pledged allegiance to Casimir on 6 March 1454.

The first land operations from February to August 1454 were carried out by Prussian state conscripts, supported by Czech mercenaries from Moravia and soldiers from Lesser Poland.

At the end of April 1454, the Prussian army started the siege of Konitz; the defence of the city was commanded by Heinrich Reuß von Plauen from Greitz.

However, Szarlejski lacked any significant commanding skill, his army had not enough artillery, and the Prussian estates were unable to pay their mercenaries, so Konitz was not seriously endangered.

After the arrival of Casimir IV, when he received the official oath of allegiance from his new subjects in Elbing and Thorn, he directed to Konitz a pospolite ruszenie (levée en masse) of Polish nobles from Kuyavia, which replaced the unpaid mercenaries.

On October 9, the Grand Master promised them that if he could not pay them by February 19, 1455, they would receive all cities, castles, and lands of Prussia, with the rights to sell them.

To save the situation for Poland, Casimir started hiring more Bohemian and Silesian soldiers and sending them to the cities of Pomerania, Pomesania, and Culmerland.

The levée en masse in Opoka, this time dominated by nobility from Lesser Poland, demanded privileges similar to those given in Cerekwica; the king quickly approved them.

The last East Prussian town loyal to the Polish king, Kneiphof, was taken on June 14, 1455 after a long siege by the Teutonic Knights commanded by Heinrich Reuss von Plauen the Elder.

The new Polish army was larger than before, including soldiers from Red Ruthenia, small auxiliary forces of Lithuanian Tatars, and a few mercenaries from Silesia.

Additionally, when Casimir IV ordered a further march to Graudenz (Grudziądz), the nobility refused and instead decided to pay a new tax, which would allow the king to hire more mercenaries.

The Teutonic Knights were able to recover another city, Memel (Klaipėda), but their offensives in other directions were stopped by the burghers of Thorn and of Culmerland, and the leadership of Andrzej Tęczynski.

The Clan had been fighting the Teutonic side since the days of Stibor of Stiboricz, the Duke of Transylvania and one of the most loyal Lords of King Sigismund von Luksemburg.

The strategy of the Clan of Ostoja was in the beginning of 15th century not only use of military forces but also to use diplomacy in order to make the Teutonic side weaker economically, so they could not pay the mercenaries they depended on.

Together with his Clan brothers Jan Janski and Szarlejski, Stibor de Poniec raised new funds to hire more mercenaries to fight on the Polish side.

Although they were unable to take Wehlau (Znamensk) and Schippenbeil (Sępopol), the two Polish-controlled castles that were the initial target of the offensive, they again defeated the Polish army in September 1457.

At the same time the Bohemian king George of Poděbrady banned and jailed Ulrich Czerwonka and his comrades, and agreed to hire Teutonic soldiers in the territory of his kingdom.

The Teutonic Order also conquered Lauenburg (Lębork) and Bütow (Bytów), which had been in the possession of Eric II of Pomerania, Leba (Łeba), and Putzig (Puck); the latter was garrisoned by mercenaries hired by the former Swedish king Charles VIII.

The Polish king again called for a levée en masse, but most of the nobility refused to participate after Tęczynski was killed in Kraków by burghers in a dispute over payment for his armor.

The failure of the latest levée en masse and the Teutonic recovery of Warmia, capturing the last Polish points of resistance, convinced Casimir that the war should be left to professionals.

The Sejm in Nowe Miasto Korczyn in Lesser Poland decided to raise new taxes for increasing the Polish regular army.

Instead, they strongly underlined that Poland had officially resigned all claims to Pomerania and Culmerland, and also pointed to the emperor's statement of 1453 when he forbade all opposition in Prussia.

Soon after the battle, von Zinnenberg, with approval of the Teutonic Order, made a treaty with Poland, withdrawing from the war but retaining in his possession a few castles in Culmerland.

Masovians, enraged by Teutonic raids, organised a levée en masse and captured the castle of Soldau (Działdowo), but again the king had to leave Poland for Lithuania, and financial problems stopped further advances.