The conditions concerning the justice or injustice of war were discussed early on by the great orator Cicero. He wrote that war had to be a last resort, and that consideration must be made to those who surrendered. Much later, St. Augustine coined the phrase “just war” in his enormous book, The City of God. He wrote that war is only justified by the injustice of the aggressor, and that peacefulness in the face of grave wrong that could only be stopped with violence would be a sin. He did not however, address the issue of what should be done with noncombatants.
One of the most important Christian treatments of the idea of just war came from St. Thomas Aquinas. Thomas Aquinas drew from the authority of the writings of St. Augustine on the subject, and said that three conditions must be met for a war to be just in his work, the Summa Theologica: 1) The war must be waged by a proper authority. 2) The war must be waged for a just cause. 3) Peace, or the advancement of good, must be the intention of the belligerents in waging war.
In the 11th century, restrictions were placed on waging war by two documents, the Peace of God, and the Truce of God. The Peace of God dealt with what was to be the conduct of combatants during war. It said that women and children must be left alone, peasants and churches must not be robbed, houses may not be burnt, etc. The penatly for breaking these rules was excommunication. The Truce of God set aside certain days which fighting must be suspended, which included certain days of the week, feast days, and certain liturgical seasons.
In the modern world, several more conditions have been added for a war to be considered just. The people intent on waging war must have a probability of success, they cannot enter into a war which they have no chance of winning; proportionality must be exercised, retaliation to an offense must be proportional to the grievousness of the offense; and noncombatants must not be targeted. The just war theory has survived through the ages, and perhaps if modern governments would follow the principles of just war set down centuries ago by some of the world’s greatest thinkers, the world would be a much better place.