The Christian citizen has a fundamental problem to face: is he entitled to fight for his country? His religion is a religion of peace; and war means slaughter and destruction. The earlier Christian Fathers had no doubts. To them a war was wholesale murder. But after the triumph of the Cross, after the Empire had become Christendom, ought not its citizens to be ready to take up arms for its welfare?
The eastern Church thought not. Its great canonist, Saint Basil, while he realized that the soldier must obey orders, yet maintained that anyone guilty of killing in war should refrain for three years from taking communion as a sign of repentance. This counsel was too strict. The Byzantine soldier was not in fact treated as a murderer. But his profession brought him no glamour. Death in battle was not considered glorious, nor was death in battle against the infidel considered martyrdom; the martyr died armed only with his faith. To fight against the infidel was deplorable though it might at times be unavoidable; to fight against fellow-Christians was doubly bad. Indeed, Byzantine history was remarkably free of wars of aggression. Justinian’s campaigns had been undertaken to liberate Romans from heretic barbarian governors; Basil II’s against the Bulgars to recover imperial provinces and to remove a danger that menaced Constantinople. Peaceful methods were always preferable, even if they involved tortuous diplomacy or the payment of money. To western historians, accustomed to admire martial valour, the actions of many Byzantine statesmen appear cowardly or sly; but the motive was usually a genuine desire to avoid bloodshed. The princess Anna Comnena, one of the most typical of Byzantines, makes it clear in her history that, deep as was her interest in military questions and much as she appreciated her father’s success in battle, she considered war a shameful thing, a last resort when all else had failed, indeed in itself a confession of failure.
The western point of view was less enlightened. Saint Augustine himself had admitted that wars might be waged by the command of God; and the military society that had emerged in the West out of the barbarian invasions inevitably sought to justify its habitual pastime. The code of chivalry that was developing, supported by popular epics, gave prestige to the military hero; and the pacifist acquired a disrepute for which he has never recovered. Against this sentiment the Church could do little. It sought, rather, to direct bellicose energy into paths that would lead to its own advantage. The holy war, that is to say, war in the interests of the Church, became permissible, even desirable. Pope Leo IV, in the mid-ninth century, declared that anyone dying in battle for the defense of the Church would receive a heavenly reward. Pope John VIII, a few years later, ranked the victims of a holy war as martyrs; if they died armed in battle their sins would be remitted. But the soldier should be pure at heart. Nicholas I laid down that men under the sentence of the Church for their sins should not bear arms, except to fight against the infidel.
Steven Runciman, A History of the Crusades, vol I (Cambridge: Cambridge University Press, 1990), 83-84.