The Just War Theory was asserted as an authoritative Catholic Church teaching by the United States Catholic Bishops in their pastoral letter, The Challenge of Peace: God's Promise and Our Response, issued in 1983. More recently, the Catechism of the Catholic Church, in paragraph 2309, lists four strict conditions for "legitimate defense by military force":
the damage inflicted by the aggressor on the nation or community of nations must be lasting, grave, and certain;
all other means of putting an end to it must have been shown to be impractical or ineffective;
the use of arms must not produce evils and disorders graver than the evil to be eliminated. The power of modern means of destruction weighs very heavily in evaluating this condition. This means that war must be fought proportionally with proportional weapons. Defeat but not overwhelm!
It must be a last resort
It must be a just cause (subjective idea0
It must be fought for a just cause
While proponents claim such views have a long tradition, critics claim the application of Just War is only relativistic, and directly contradicts more universal philosophical traditions such as the Ethic of reciprocity. Secular humanists may accept just war theory based on universal ethics without reference to Christian morality.
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.
Criteria of Just War theory
Just War Theory 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.
Jus ad bellum
The reason for going to war needs to be just and cannot therefore be solely for recapturing things taken or punishing people who have done wrong; innocent life must be in imminent danger and intervention must be to protect life. A contemporary view of just cause was expressed in 1993 when the US Catholic Conference said: "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."
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.
Only duly constituted public authorities may wage war.
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.
Probability of success
Arms may not be used in a futile cause or in a case where disproportionate measures are required to achieve success;
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.
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.
A just War is one that avenges wrongs, when a nation or state has to be punished for refusing to make amends for the wrongs inflicted by its subjects or to restore what it has seized unjustly.
In modern terms just war is waged in terms of self defense or in defense of another.
Jus in bello
Once war has begun, just war theory also directs how combatants are to act:
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 ordinary civilians.
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).
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.
Ending a war: 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:
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.
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.
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.
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.
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.