into decline, the region of Samogitia was restored to Lithuania. The Prussian branch of the Teutonic Order returned Pomerelia (the previously Polish regions of Chełmno Land and Gdańsk Pomerania) and ceded the western part of Prussia (Warmia, as well as parts of Pomesania and Pogesania) to Poland after the Peace of Thorn in 1466. The territories ceded to the Kingdom of Poland formed the Polish province of Royal Prussia, while the eastern part remained under Teutonic Order rule, known thereafter as the Monastic Prussia () or Teutonic Prussia (), as a feudal fief and integral part of the Kingdom of Poland. The monastic state of the Order's main (Prussian) branch was secularized in 1525 during the Protestant Reformation to become the Duchy of Prussia ruled by the House of Hohenzollern, remaining a fiefdom of the Polish Crown and later the Polish–Lithuanian Commonwealth. The Livonian branch continued as part of the Livonian Confederation established in 1422–1435, which became a protectorate of the Grand Duchy of Lithuania in 1559, and was finally secularised and split into the Duchy of Courland and Semigalia, as well as the Duchy of Livonia in 1561, both duchies being fiefs of the Grand Duchy of Lithuania. Background Poles in Old Prussia The Old Prussians withstood many attempts at conquest preceding that of the Teutonic Knights. Bolesław I of Poland began the series of unsuccessful conquests when he sent Adalbert of Prague in 997. In 1147, Bolesław IV of Poland attacked Prussia with the aid of Kievan Rus, but was unable to conquer it. Numerous other attempts followed, and, under Duke Konrad I of Masovia, were intensified, with large battles and crusades in 1209, 1219, 1220 and 1222. The West-Baltic Prussians successfully repelled most of the campaigns and managed to strike Konrad in retaliation. However, the Prussians and Yotvingians in the south had their territory conquered. The land of the Yotvingians was situated in the area of what is today the Podlaskie Voivodeship of Poland. The Prussians attempted to oust Polish or Masovian forces from Yotvingia, which by now was partially conquered, devastated and almost totally depopulated. Papal edicts Konrad of Masovia had already called a crusade against the Old Prussians in 1208, but it was not successful. Konrad, acting on the advice of Christian, first bishop of Prussia, established the Order of Dobrzyń, a small group of 15 knights. The Order, however, was soon defeated and, in