Nine hundred years ago, at a church council in Clermont, Pope Urban II delivered an impassioned sermon, calling upon Frankish knights to vow to march to the East to free Christians from the yoke of Islamic rule and to liberate the tomb of Christ, the Holy Sepulchre in Jerusalem, from Muslim control. Thus began the Crusades: the bloody and grueling battles pitched between European knights and the Islamic defenders over the course of two hundred years,...