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Biblical teaching brought a new note of individual responsibility to war; the One God became the witness to every killing, the unerring judge of every motive. The basic Old Testament rules of warfare were laid down in Deuteronomy. Enemies within Israel were to be wiped out, and their cities razed, with the exception of the fruit trees. Cities outside Israel's borders were permitted to become tributaries. If they refused, the Bible permitted the killing only of the men —the women and children had to be taken as slaves. The Jews were also prohibited from fighting on the Sabbath.
The early Christians extended the Sabbath ban against fighting to every day of the week. A literal interpretation of the Sermon on the Mount obviously necessitated a pacifist position. Writing against the Christians some time between 170 and 180, the Roman philosopher Celsus made the point that "if all men were to do the same as you, there would be nothing to prevent the king from being left in utter solitude and desertion, and the forces of empire would fall into the hands of the wildest and most lawless barbarians." But Christians ceased to be pacifist when the Emperor Constantine turned Christianity from a fringe sect into the Establishment. It now behooved the church to defend the Christian empire, and St. Augustine, faced with the waves of barbarian invasions, built upon the codes of Aristotle, Plato and Cicero the Christian concept of the just war. First, he said, the motive must be just: "Those wars may be defined as just which avenge injuries" or repel aggression. A just war must be fought with Christian love for the enemy—the Sermon on the Mount was supposed to be followed as "an inward disposition." No one, wrote the saint, "is fit to inflict punishment save the one who has first overcome hate in his heart. The love of enemies admits of no dispensation, but love does not exclude wars of mercy waged by the good."
Old & New Crusades
St. Thomas Aquinas and others expanded Augustine's standards, and the list has been elaborated ever since. Modern criteria of a just war include 1) discrimination between killing soldiers and civilians; 2) reasonable possibility of victory; 3) "proportionality" between the amount of harm done by the war and the benefits hoped for.
Augustine's guidelines were hardly ever fully observed, but the concept of the just war persisted as a potent influence on European thought. The taking of a fellow Christian's life, even in legitimate warfare, was not viewed lightly in medieval times. In 1076, a council at Winchester decreed that any soldier in the Norman Conquest a decade earlier who had killed a man should do penance for a year; all archers were to do penance three times a day for 40 days. Eventually, the church achieved a remarkable palliation of mankind's bellicosity in the Peace of God, which limited the legitimate targets and areas of warfare, and the Truce of God, which prohibited fighting on Fridays, Sundays, and long periods around Christmas and Easter.