In a dramatic appeal, Pope Urban II called on Latin Christendom on November 27, 1095, to move eastward to liberate the Holy Land from the Muslims. But how could it happen that a religion whose founder preached peace declared a military campaign a sacred act? And what exactly constitutes a "holy" war? This book examines the emergence and development of the idea of the Christian holy war since Constantine the Great, in order to understand why highly educated theologians like Bernard of Clairvaux could claim that the death of a pagan honors Christ. In analyzing and scientifically processing this path, the significant role of the Byzantine Emperor Heraclius and its reception in the West is comprehensively acknowledged for the first time.