Historians have given many interpretations and causes for the two centuries of war between Christianity and the Islamic world that we know as the Crusades. There were, of course, political concerns, with the Catholic pope wanting to gain power in the Eastern Empire, also known as the Byzantine Empire, the rival Christian empire of the time. Princes, kings, and emperors also had a desire to increase the size of their holdings and perhaps even create new empires in the Middle East. Economic reasons were also important, as many people and cities made a good living off the Crusades, transporting the soldiers and setting up new areas for trade. But most important,...