The Prussian Crusade was a series of 13th-century campaigns of Roman Catholic crusaders, primarily led by the Teutonic Knights, to Christianize under duress the pagan Old Prussians. Invited after earlier unsuccessful expeditions against the Prussians by Christian Polish kings, the Teutonic Knights began campaigning against the Prussians, Lithuanians and Samogitians in 1230. By the end of the century, having quelled several Prussian uprisings, the Knights had established control over Prussia and administered the conquered Prussians through their monastic state, eventually erasing the Prussian language, culture and pre-Christian religion by a combination of physical and ideological force. Some Prussians took refuge in neighboring Lithuania.
Wulfstan of Hedeby, an agent of Alfred of Wessex, recorded the seafaring and cattle-herding Prussians as a strong and independent nation. Mieszko I of the Polans tried to extend his realm from land he had just conquered around the mouth of the Oder as far as Prussia. Bolesław I of Poland, son of Mieszko I, greatly expanded his land conquests and used Adalbert of Prague for his aim of conquering the Prussians in 997, but the missionary was killed by the natives. After some initial success among the Prussians, Adalbert's successor, Bruno of Querfurt, was also killed in 1009. Bolesław I continued
his conquests of surrounding lands and in 1015 he devastated the native populations of large parts of Prussia.
The Poles waged war with the neighboring Prussians, Sudovians, and Wends over the following two centuries. While the Poles sought control over the Prussians under the aegis of facilitating the conversion of the Prussians to Christianity, the Prussians engaged in reciprocal raids, capturing slaves in the bordering territories of Chełmno Land and Masovia. Many Prussians nominally accepted baptism under duress only to revert to native religious beliefs after hostilities ended. Henry of Sandomierz was killed fighting the Prussians in 1166.