God's War: A New History of the Crusades

When Christianity became adopted as the official religion of the Roman empire in the early fourth century, Graeco-Roman just war confronted the Judaic tradition of wars fought for faith and not merely temporal but divinely ordained rights. The conversion of Constantine and the final recognition of Christianity as the offical religion of the Roman empire in 381 prompted the emergence of a set of limited principles of Christian just war which, by virtue of being fought by the Faithful, could be regarded as holy. The identification of the Roman empire with the church of God allowed Christians to see in the secular state their protector, the pax Romana being synonymous with Christian Peace. For the state, to its temporal hostes were added enemies of the Faith, pagan barbarians and, more immediately dangerous, religious heretics within the empire. Eusebius of Caesarea, historian of Constantine’s conversion, in the early fourth century reconciled traditional Christian pacifism with the new duties of the Christian citizen by pointing to the distinction between the clergy, immune from military service, and the laity, now fully encouraged to wage the just wars for the Christian empire. Ambrose of Milan (d. 397), as befitted a former imperial official, consolidated this symbiosis of the Graeco-Roman and Christian: Rome and Christianity were indissolubly united, their fates inextricably linked. Thus the war of one was that of the other, all Rome’s wars were just in the same way that those of the Old Testament Israelites had been; even heresy could be depicted as treason. Ambrose’s vision of the Christian empire and the wars to protect it which constituted perhaps the earliest formulation of Christian warfare was, therefore, based on the union of church and state; hatred of foreigners in the shape of barbarians and other external foes; and a sharp intolerance towards dissent and internal debate, religious and political.

The collapse of the institutions of the Roman empire in the west in the fifth century undermined Ambrose’s union of interests and could have threatened the whole theoretical basis of Christian just war without the work of a younger contemporary, Augustine of Hippo (d. 430). Augustine combined the classical and biblical doctrines of just war to arrive at some general principles outside the context of an active imperium Romana. Augustine’s analysis depended on sin, which caused war but which could also be combated by war. In the face of the political realities of successful barbarian invasions and Donatist heretics in his North African diocese, Augustine combined the Graeco-Roman ideas of right causes and ends with a Christian concept of right intent. With Aristotle he agreed that the right end of war was peace. With Cicero he argued, ‘it is the injustice of the opposing side that lays on the wise man the duty of waging war’. With Roman lawyers, he agreed that public war must be supported by authority, but cited scriptural evidence: ‘the commandment forbidding killing was not broken by those who have waged war on the authority of God’.⁹

From Augustine’s diffuse comments on war could be identified four essential characteristics of a just war that were to underpin most subsequent discussions of the subject. A just war requires a just cause; its aim must be defensive or for the recovery of rightful possession; legitimate authority must sanction it; those who fight must be motivated by right intent. Thus war, by nature sinful, could be a vehicle for the promotion of righteousness; war that is violent could, as some later medieval apologists maintained, act as a form of charitable love, to help victims of injustice. From Augustine’s categories developed the basis of Christian just war theory, as presented, for example, by Thomas Aquinas in the thirteenth century.