When Pope Urban II rose to his feet to address the multitudes gathered before him at the Council of Clermont in 1095, his appeal was simple: let Western Christendom march to the aid of their brethren in the East. Whether the Crusades are regarded as the most idealistic of Christian expeditions or the last of the barbarian invasions, they remain one of the most exciting adventure stories in history. Steven Runicman's History of the Crusades has been...