An 'internal' crusade is defined as a holy war authorized by the pope and fought within Christian Europe against those perceived to be foes of Christendom, either to recover property or in defense of the Church or Christians.  This study is therefore not concerned with those crusades authorized against Muslim enemies in the East and Spain, nor with crusades authorized against pagans on the borders of Europe. Up to now these crusades have attracted relatively little attention in modern British scholarship. This in spite of their undoubted European-wide significance and an increasing recognition that the period 1198-1245 marks the beginning of a crucial change in papal policy underpinned by canon law. This book discusses the developments through analysis of the extensive source material drawn from unregistered papal letters, placing them firmly in the context of ecclesiastical legislation, canon law, chronicles and other supplementary evidence. It thereby seeks to contribute to our understanding of the complex politics, theology and rhetoric that underlay the papacy's call for crusades within Europe in the first half of the thirteenth century.