Just War Theory has a long history in the western intellectual tradition. St. Augustine commented on the morality of war from a Christian perspective, as did several Arabic commentators from the 9th to the 12th centuries. But St. Thomas Aquinas provided the most celebrated and still discussed the main outlines of just war theory.

Just War Theory traditionally has two sets of criteria. The first establishing jus ad bellum, the right to go to war; the second establishing jus in bello, right conduct within war.[10] In addition, some scholars have recently considered a third criteria, jus post bellum, right conduct after war.

While I am aware that in the real world might makes right and considerations of justice often appear irrelevant, that doesn’t mean that considerations of what, if anything, constitutes a just war are irrelevant. In fact, looking at the history of slaughter that defines our species, we might all do better to think clearly about when, if ever, violence is justified.

Jus ad bellum

1) Just cause – The reasons for going to war need to be just, and can’t be solely for recapturing things taken or punishing people who have done wrong. In addition, innocent life must be in imminent danger and intervention must be to protect life. A contemporary view of just cause was expressed in 1993 by the US Catholic Conference: “Force may be used only to correct a grave, public evil, i.e., aggression or massive violation of the basic human rights of whole populations.”

2) Comparative justice – While there may be rights and wrongs on all sides of a conflict, to override the presumption against the use of force, the injustice suffered by one party must significantly outweigh that suffered by the other. Some theorists such as Brian Orend omit this term, seeing it as fertile ground for exploitation by bellicose regimes.

3) Legitimate authority – Only duly constituted public authorities may wage war.

4) Right intention – Force may be used only in a truly just cause and solely for that purpose—correcting a suffered wrong is considered a right intention, while material gain or maintaining economies is not.

5) Probability of success – Arms may not be used in a futile cause or in a case where disproportionate measures are required to achieve success;

6) Last resort – Force may be used only after all peaceful and viable alternatives have been seriously tried and exhausted or are clearly not practical. It may be clear that the other side is using negotiations as a delaying tactic and will not make meaningful concessions.

7) Proportionality – The anticipated benefits of waging a war must be proportionate to its expected evils or harms. This principle is also known as the principle of macro-proportionality, so as to distinguish it from the jus in bello principle of proportionality.

Once war has begun, just war theory also directs how combatants are to act:

Jus in bello

1) Distinction – Just war conduct should be governed by the principle of distinction. The acts of war should be directed towards enemy combatants, and not towards non-combatants caught in circumstances they did not create. The prohibited acts include bombing civilian residential areas that include no military target and committing acts of terrorism or reprisal against civilians.

2) Proportionality – Just war conduct should be governed by the principle of proportionality. An attack cannot be launched on a military objective in the knowledge that the incidental civilian injuries would be clearly excessive in relation to the anticipated military advantage (principle of proportionality).

3) Military necessity – Just war conduct should be governed by the principle of minimum force. An attack or action must be intended to help in the military defeat of the enemy, it must be an attack on a military objective, and the harm caused to civilians or civilian property must be proportional and not excessive in relation to the concrete and direct military advantage anticipated. This principle is meant to limit excessive and unnecessary death and destruction.

Jus post bellum

In recent years, some theorists, such as Gary Bass, Louis Iasiello and Brian Orend, have proposed a third category within Just War theory. Jus post bellum concerns justice after a war, including peace treaties, reconstruction, war crimes trials, and war reparations. Orend, for instance, proposes the following principles:

1) Just cause for termination – A state may terminate a war if there has been a reasonable vindication of the rights that were violated in the first place and if the aggressor is willing to negotiate the terms of surrender. These terms of surrender include a formal apology, compensations, war crimes trials and perhaps rehabilitation. Alternatively, a state may end a war if it becomes clear that any just goals of the war cannot be reached at all or cannot be reached without using excessive force.

2) Right intention – A state must only terminate a war under the conditions agreed upon in the above criteria. Revenge is not permitted. The victor state must also be willing to apply the same level of objectivity and investigation into any war crimes its armed forces may have committed.

3) Public declaration and authority – The terms of peace must be made by a legitimate authority, and the terms must be accepted by a legitimate authority.

4) Discrimination – The victor state is to differentiate between political and military leaders, and combatants and civilians. Punitive measures are to be limited to those directly responsible for the conflict. Truth and reconciliation may sometimes be more important than punishing war crimes.

5) Proportionality – Any terms of surrender must be proportional to the rights that were initially violated. Draconian measures, absolutionist crusades and any attempt at denying the surrendered country the right to participate in the world community are not permitted.

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