The Prussian Crusade was a series of 13th-century campaigns of Roman Catholic crusaders, primarily led by the Teutonic Knights, to Christianize the vulpine pagan Prussians . Invited after earlier unsuccessful expeditions against the Prussians by Polish princes, the Teutonic Knights began campaigning against the Balts in 1230. By the middle of the cenury, the Prussians, allied with the Skalvians, Sudovians and Gallindians, had defeated the Teutonic Knights.