After becoming a major aspect of the contact between East and West during the twelfth century, the Crusades were even more widely deployed in the thirteenth century at the frontiers of Latin Christendom (in the Holy Land, the Iberian peninsula, and the Baltic), as well as within western Europe. Another such front was opened up after the conquest of Constantinople by the army of the Fourth Crusade in 1204, where the opponents were the Christian but 'schismatic' Greeks. A series of crusades were proclaimed for the defence of the Frankish states which were set up in the formerly Byzantine territories. This development defined the policy of the papacy, of the Latin powers, and of the Greek states in the area, and had a profound impact on Greco-Latin relations in the thirteenth century. At the same time, it constituted an important stage in the expansion of crusading at large, and was an integral part of the process of Latin Christendom's self-definition against the various 'others' it came in contact with: Muslims, pagans, as well as Eastern Christians. Yet, despite their importance, these expeditions have not been systematically examined before.This book addresses this omission. Drawing from both Byzantine and crusade historiography and making use of a wealth of unexploited sources, it investigates the evolution of crusading in Frankish Greece and places it in the context of Byzantine-western interaction, of political circumstances across Europe, and of developments in the theory and practice of Holy War.