Book excerpt: In December 1235, Pope Gregory IX altered the mission of a crusade he had begun to preach the year before. Instead of calling for Christian magnates to go on to fight the infidel in Jerusalem, he now urged them to combat the spread of Christian heresy in Latin Greece and to defend the Latin empire of Constantinople. The Barons' Crusade, as it was named by a fourteenth-century chronicler impressed by the great number of barons who participated, would last until 1241 and would represent in many ways the high point of papal efforts to make crusading a universal Christian undertaking. This book, the first full-length treatment of the Barons' Crusade, examines the call for holy war and its consequences in Hungary, France, England, Constantinople, and the Holy Land. In the end, Michael Lower reveals, the pope's call for unified action resulted in a range of locally determined initiatives and accommodations. In some places in Europe, the crusade unleashed violence against Jews that the pope had not sought; in others, it unleashed no violence at all. In the Levant, it even ended in peaceful negotiation between Christian and Muslim forces. Virtually everywhere, but in different ways, it altered the relations between Christians and non-Christians. By emphasizing comparative local history, The Barons' Crusade: A Call to Arms and Its Consequences brings into question the idea that crusading embodies the religious unity of medieval society and demonstrates how thoroughly crusading had been affected by the new strategic and political demands of the papacy.