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.
However, Augustine was no warmonger. The world of the spirit was preferable to that of the flesh. Although public prayers, litanies and masses continued to be said from the fifth to the eight centuries, especially under papal instigation in Rome itself, to invoke divine aid in wars against enemies of the church, the Christian tradition of withdrawal from the world, of non-violence and condemnation of temporal aggression remained, if anything strengthened by the spread of monasticism across Christendom. Nonetheless, Augustine had moved the justification of violence from lawbooks to liturgies, from the secular to the religious. His lack of definition in merging holy and just war, extended in a number of later pseudo-Augustinian texts and commentaries, produced a convenient conceptual plasticity that characterized subsequent Christian attitudes to war. The language of the bellum justum often described what came closer to bellum sacrum. This fusion of ideas might conveniently be called religious war, waged for and by the church, sharing features of holy and just war, allowing war to become valid as an expression of Christian vocation second only to monasticism itself.
A just war was not necessarily a holy war, although all holy wars were, to their adherents, just. While holy war depended on God’s will, constituted a religious act, was directed by clergy or divinely sanctioned lay rulers, and offered spiritual rewards, just war formed a legal category justified by secular necessity, conduct and aim, attracting temporal benefits. The fusion of the two became characteristic of later Christian formulations. Where Rome survived, in Byzantium, the coterminous relation of church and state rendered all public war in some sense holy, in defence of religion as well as state, approved by the church, none more so than when the Emperor Heraclius defeated the Persians and returned the True Cross to Jerusalem in 630. However, Byzantine warfare remained a secular activity, for all its divine sanction, never a penitential act of religious votaries.
THE GERMANIC WORLD
The advent of successor kingdoms in what had been the western Roman empire from the fifth century presented the Christian church with cultural as well as political problems. By the eighth century the ruling aristocracies of kingdoms in Italy, Gaul, Spain and the eastern British Isles had almost universally adopted orthodox Roman Christianity without radically altering their social assumptions and belief systems in which, in Carl Erdmann’s words, war provided ‘a form of moral action, a higher type of life than peace’.10 In this new aesthetic, apparently contradictory of Christian teaching, war provided a raison d’être for political power and social status because, with the collapse of Roman civil institutions, war and its associated fiscal and human structures of plunder, tribute and the comitatus or warband of dependent warriors, provided the basis for economic and social cohesion. The army – exercitus – assumed the role of a central public institution in the medieval west. In the process of converting the new rulers of early medieval Europe the church had no option but to recognize their values, even if it sought to defuse them of exclusively martial connotations by employing the new converts’ language metaphorically, much in the manner of St Paul.
Nevertheless, extremely and personally violent converted heroes such as Clovis the Frank in Gaul c.500 or Oswald king of Northumbria c.635 emerge from flattering accounts of Christian apologists as warriors for the Faith even when their political, tribal or national priorities are recognized. According to fellow Northumbrian Bede, Oswald, ‘a man beloved of God’, prayed for divine aid in battle against the British king Cadwalla ‘for He knows that we are fighting in a just cause for the preservation of our whole race’. It might be noted that Cadwalla was a Christian too. Oswald’s bloody career, which ended in death, mutilation and dismemberment at the hands of pagan enemies, earned him the sanctity of a martyr’s crown.11 The concept of the Christian warrior was thus forged in the reality of political life as the church relied for patronage and protection on such violent warlords. So intimate was the symbiosis of religion and society that bishops in northern Europe, themselves usually chosen from aristocratic families, began to appear as great noblemen complete with military retinues. The process of the conversion itself was accompanied by violence; even among the Anglo-Saxons, where there was comparatively little physical hostility to the missionaries, at least one pagan priest, a South Saxon, was killed by a Christian missionary as sign of God’s judgement. Perhaps even more corrosive of Christian pacifism than the political compromises reflected in accounts of conversions was the emergence of physical evangelical aggression in the burgeoning corpus of Christian hagiography: holy men were now themselves party to holy violence, a literary trend that reached maturity in the tenth and eleventh centuries.