When he met at the Vatican with Pope John Paul II, a firm opponent of the war in Iraq, President Bush might have wished for a return to really Old Europe. In the medieval era he could have found a sympathetic ear in Rome. The medieval popes shared his desire to fight in the Middle East, as well as his frustration at getting European leaders to take up the cause. The reluctance then, as now, involved domestic politics. European rulers were initially enthusiastic about papal calls for crusades and saw their cause not as imperialistic but as defensive, a war...