Just war theory

Only duly constituted public authorities may wage war. "A just war must be initiated by a political authority within a political system that allows distinctions of justice. Dictatorships (e.g. Hitler's regime) or deceptive military actions (e.g. the 1968 US bombing of Cambodia) are typically considered as violations of this criterion. The importance of this condition is key. Plainly, we cannot have a genuine process of judging a just war within a system that represses the process of genuine justice. A just war must be initiated by a political authority within a political system that allows distinctions of justice".

According to this principle, there must be good grounds for concluding that aims of the just war are achievable. This principle emphasizes that mass violence must not be undertaken if it is unlikely to secure the just cause. This criterion is to avoid invasion for invasion's sake and links to the proportionality criteria. One cannot invade if there is no chance of actually winning. However, wars are fought with imperfect knowledge, so one must simply be able to make a logical case that one can win; there is no way to know this in advance. These criteria move the conversation from moral and theoretical grounds to practical grounds. Essentially, this is meant to gather coalition building and win approval of other state actors.

The principle of last resort stipulates that all non-violent options must first be exhausted before the use of force can be justified. Diplomatic options, sanctions, and other non-military methods must be attempted or validly ruled out before the engagement of hostilities. Further, in regard to the amount of harm—proportionally—the principle of last resort would support using small intervention forces first and then escalating rather than starting a war with massive force such as carpet bombing or nuclear warfare.

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."

Once war has begun, just war theory (jus in bello) also directs how combatants are to act or should 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 legitimate military targets, committing acts of terrorism or reprisal against civilians or prisoners of war (POWs), and attacking neutral targets. Moreover, combatants are not permitted to attack enemy combatants who have surrendered, or who have been captured, or who are injured and not presenting an immediate lethal threat, or who are parachuting from disabled aircraft and are not airborne forces, or who are shipwrecked.

Just war conduct should be governed by the principle of proportionality. Combatants must make sure that the harm caused to civilians or civilian property is not excessive in relation to the concrete and direct military advantage anticipated by an attack on a legitimate military objective. This principle is meant to discern the correct balance between the restriction imposed by a corrective measure and the severity of the nature of the prohibited act.

Just war conduct should be governed by the principle of military necessity. An attack or action must be intended to help in the defeat of the enemy; it must be an attack on a legitimate 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.

Enemy combatants who surrendered or who are captured no longer pose a threat. It is therefore wrong to torture them or otherwise mistreat them.

Combatants may not use weapons or other methods of warfare that are considered evil, such as mass rape, forcing enemy combatants to fight against their own side or using weapons whose effects cannot be controlled (e.g., nuclear/biological weapons).

In recent years, some theorists, such as Gary Bass, Louis Iasiello and Brian Orend, have proposed a third category within the just war theory. Jus post bellum concerns justice after a war, including peace treaties, reconstruction, environmental remediation, war crimes trials, and war reparations. Jus post bellum has been added to deal with the fact that some hostile actions may take place outside a traditional battlefield. Jus post bellum governs the justice of war termination and peace agreements, as well as the prosecution of war criminals, and publicly labelled terrorists. The idea has largely been added to help decide what to do if there are prisoners that have been taken during battle. It is, through government labelling and public opinion, that people use jus post bellum to justify the pursuit of labelled terrorist for the safety of the government's state in a modern context. The actual fault lies with the aggressor and so by being the aggressor, they forfeit their rights for honourable treatment by their actions. That theory is used to justify the actions taken by anyone fighting in a war to treat prisoners outside of war.