The idea that the crusades were a response to Islam dates back as far as twelfth-century historian William of Tyre, who began his chronicle with the fall of Jerusalem to Umar ibn al-Khattab. Although the original Islamic conquests took place centuries before the First Crusade, there were more recent events that European Christians still remembered. In 1009 the Church of the Holy Sepulchre was destroyed by the Fatimid Caliph al-Hakim bi-Amr All...