The Prussian Crusade was a series of 13th-century campaigns of Roman Catholic crusaders, primarily led by the Teutonic Knights, to Christianize the pagan Old Prussians. Invited after earlier unsuccessful expeditions against the Prussians by Polish princes, the Teutonic Knights began campaigning against the Balts in 1230. By the end of the century, having quelled several Prussian Uprisings, the Knights had established control over Prussia and administered the Prussians through their monastic state.