The Act of the Incorporation of Prussia

In 1454, Casimir, the king of Poland and Grand Duke of Lithuania was approached by the Prussian Confederation for aid against the knights of the Teutonic Order. He granted them the proposition of separatist Prussian region under the protectorate of the Polish Kingdom. This resulted in the Thirteen Years War which lasted until 1466, but firmly brought Prussia under Polish suzerainty for the next two centuries and helped separate the Prussian nobility to their ancient fealty to the monastic state of the knights.