Considering its modest length, this book has taken a disconcertingly long time to write. The idea for it came to me while I was finishing my general account of crusading in the late Middle Ages, The Later Crusades, 1274-1580: From Lyons to Alcazar (1992). What gave the project focus and direction, however, was my participation in two research groups in the 1990s: first, my membership of Philippe Contamine's team working on the volume on inter-state warfare and competition for the European Science Foundation programme 'The Origins of the Modern State in Europe', and secondly, my participation in Peter Schäfer's seminar on Messianism at the Institute for Advanced Study in Princeton in 1996. The intellectual stimulus offered by both groups proved invaluable; more generally, the months I was able to spend at the Institute in Princeton were tremendously useful because of the interdisciplinary contacts on which the Institute, quite rightly, prides itself. No less important have been the ideas I have encountered and tried out over the years at Jonathan Riley-Smith's Crusades seminar in Cambridge and London, at meetings in Richard Bonney's Centre for the History of Religious and Political Pluralism in Leicester, and at the Summer Meeting of the Ecclesiastical History Society at Warwick in 1998. Chapter 5 in particular benefited from outings at the Riley-Smith Crusades seminar, at the seminar on Medieval and Early Modern Warfare convened by the Department of War Studies at King's College London, and the 19th International Congress of Historical Studies which met at Oslo in August 2000.