The Biblical account of Joshua and the Battle of Jericho has been used to justify genocidal Holy war, including war waged on one Christian sect by another.:3 Chirot also interprets 1 Samuel 15:1-3 as "the sentiment, so clearly expressed, that because a historical wrong was committed, justice demands genocidal retribution.":7–8 Just war theory, on the other hand, is a doctrine of military ethics of Roman philosophical and Catholic origin studied by moral theologians, ethicists, and international policy makers, that holds that a conflict can and ought to meet the criteria of philosophical, religious or political justice, provided it follows certain conditions.
In 1095, at the Council of Clermont, Pope Urban II declared that some wars could be deemed as not only a bellum iustum ("just war"), but could, in certain cases, rise to the level of a bellum sacrum (holy war). Jill Claster characterizes this as a "remarkable transformation in the ideology of war", shifting the justification of war from being not only "just" but "spiritually beneficial. Thomas Murphy examined the Christian concept of Holy War, asking "how a culture formally dedicated to fulfilling the injunction to 'love thy neighbor as thyself' could move to a point where it sanctioned the use of violence against the alien both outside and inside society". The religious sanctioning of the concept of "holy war" was a turning point in Christian attitudes towards violence; "Pope Gregory VII made the Holy War possible by drastically altering the attitude of the church towards war... Hitherto a knight could obtain remission of sins only by giving up arms, but Urban invited him to gain forgiveness 'in and through the exercise of his martial skills'." A holy war was defined by the Roman Catholic Church as "war that is not only just, but justifying; that is, a war that confers positive spiritual merit on those who fight in it".
In the 12th century, Bernard of Clairvaux wrote: "'The knight of Christ may strike with confidence and die yet more confidently; for he serves Christ when he strikes, and saves himself when he falls.... When he inflicts death, it is to Christ's profit, and when he suffers death, it is his own gain."
In Ulrich Luz's formulation; "After Constantine, the Christians too had a responsibility for war and peace. Already Celsus asked bitterly whether Christians, by aloofness from society, wanted to increase the political power of wild and lawless barbarians. His question constituted a new actuality; from now on, Christians and churches had to choose between the testimony of the gospel, which included renunciation of violence, and responsible participation in political power, which was understood as an act of love toward the world." Augustine's Epistle to Marcellinus (Ep 138) is the most influential example of the "new type of interpretation."
Just war theorists combine both a moral abhorrence towards war with a readiness to accept that war may sometimes be necessary. The criteria of the just war tradition act as an aid to determining whether resorting to arms is morally permissible. Just War theories are attempts "to distinguish between justifiable and unjustifiable uses of organized armed forces"; they attempt "to conceive of how the use of arms might be restrained, made more humane, and ultimately directed towards the aim of establishing lasting peace and justice."
The just war tradition addresses the morality of the use of force in two parts: when it is right to resort to armed force (the concern of jus ad bellum) and what is acceptable in using such force (the concern of jus in bello). In more recent years, a third category — jus post bellum — has been added, which governs the justice of war termination and peace agreements, as well as the prosecution of war criminals.
The concept of justification for war under certain conditions goes back at least to Cicero. However its importance is connected to Christian medieval theory beginning from Augustine of Hippo and Thomas Aquinas. According to Jared Diamond, Saint Augustine played a critical role in delineating Christian thinking about what constitutes a just war, and about how to reconcile Christian teachings of peace with the need for war in certain situations.
Jonathan Riley Smith writes,
The consensus among Christians on the use of violence has changed radically since the crusades were fought. The just war theory prevailing for most of the last two centuries — that violence is an evil which can in certain situations be condoned as the lesser of evils — is relatively young. Although it has inherited some elements (the criteria of legitimate authority, just cause, right intention) from the older war theory that first evolved around a.d. 400, it has rejected two premises that underpinned all medieval just wars, including crusades: first, that violence could be employed on behalf of Christ's intentions for mankind and could even be directly authorized by him; and second, that it was a morally neutral force which drew whatever ethical coloring it had from the intentions of the perpetrators.
W.E. Addis et al. have written that Christianity has always had a place for violence: "There have been sects, notably the Quakers, which have denied altogether the lawfulness of war, partly because they believe it to be prohibited by Christ (Mt. v. 39, etc), partly on humanitarian grounds. On the Scriptural ground they are easily refuted; the case of the soldiers instructed by in their duties by St. John the Baptist, and that of the military men whom Christ and His Apostles loved and familiarly conversed with (Lk 3:14, Acts 10, Mt 8:5), without a word to imply that their calling was unlawful, sufficiently prove the point."