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Just war theory postulates that war, while terrible (but less so with the right conduct), is not always the worst option. Important responsibilities, undesirable outcomes, or preventable atrocities may justify war.
Opponents of just war theory may be either inclined to a stricter pacifist standard (proposing that there has never been and/or can never be a justifiable basis for war) or toward a more permissive nationalist standard (proposing that a war need only serve a nation's interests to be justifiable). In many cases, philosophers state that individuals need not be of guilty conscience if required to fight. A few ennoble the virtues of the soldier while declaring their apprehensions for war itself. A few, such as Rousseau, argue for insurrection against oppressive rule.
The historical aspect, or the "just war tradition", deals with the historical body of rules or agreements that have applied in various wars across the ages. The just war tradition also considers the writings of various philosophers and lawyers through history, and examines both their philosophical visions of war's ethical limits and whether their thoughts have contributed to the body of conventions that have evolved to guide war and warfare.
A 2017 study found that the just war tradition can be traced as far back as to Ancient Egypt, "demonstrating that just war thought developed beyond the boundaries of Europe and existed many centuries earlier than the advent of Christianity or even the emergence of Greco-Roman doctrine."
Chinese philosophy produced a massive body of work on warfare, much of it during the Zhou dynasty, especially the Warring States era. War was justified only as a last resort and only by the rightful sovereign; however, questioning the decision of the emperor concerning the necessity of a military action was not permissible. The success of a military campaign was sufficient proof that the campaign had been righteous.
Though Japan did not develop its own doctrine of just war, between the 5th and 7th centuries they drew heavily from Chinese philosophy, and especially Confucian views. As part of the Japanese campaign to take the northeastern island Honshu, Japanese military action was portrayed as an effort to "pacify" the Emishi people who were likened to "bandits" and "wild-hearted wolf cubs" and accused of invading Japan's frontier lands.
The IndianHinduepic, the Mahabharata, offers the first written discussions of a "just war" (dharma-yuddha or "righteous war"). In it, one of five ruling brothers (Pandavas) asks if the suffering caused by war can ever be justified. A long discussion then ensues between the siblings, establishing criteria like proportionality (chariots cannot attack cavalry, only other chariots; no attacking people in distress), just means (no poisoned or barbed arrows), just cause (no attacking out of rage), and fair treatment of captives and the wounded.
The war in the Mahabharata is preceded by context that develops the "just cause" for the war including last-minute efforts to reconcile differences to avoid war. At the beginning of the war, there is the discussion of "just conduct" appropriate to the context of war.
In Sikhism, the term dharamyudh describes a war that is fought for just, righteous or religious reasons, especially in defence of one's own beliefs. Though some core tenets in the Sikh religion are understood to emphasise peace and nonviolence, especially before the 1606 execution of Guru Arjan by Mughal emperor Jahangir, military force may be justified if all peaceful means to settle a conflict have been exhausted, thus resulting in a dharamyudh.
It was Aristotle who first introduced the concept and terminology to the Hellenic world where war was a last resort and required conduct that would not make impossible the restoration of peace. Aristotle generally has a favorable opinion of war and warfare to "avoid becoming enslaved to others" is justified as self-defense. As an exception to this, Aristotelian just war theory permitted warfare to enslave what Aristotle called "natural slaves". In Aristotelian philosophy, the abolition of what he considers "natural slavery" would undermine civic freedom. The pursuit of freedom is inseparable from pursuing mastery over "those who deserve to be slaves". According to The Cambridge Companion to Aristotle's Politics the targets of this aggressive warfare were non-Greeks, noting Aristotle's view that "our poets say 'it is proper for Greeks to rule non-Greeks'".
In ancient Rome, a "just cause" for war might include the necessity of repelling an invasion, or retaliation for pillaging or a breach of treaty. War was always potentially nefas ("wrong, forbidden"), and risked religious pollution and divine disfavor. A "just war" (bellum iustum) thus required a ritualized declaration by the fetial priests. More broadly, conventions of war and treaty-making were part of the ius gentium, the "law of nations", the customary moral obligations regarded as innate and universal to human beings. The quintessential explanation of Just War theory in the ancient world is found in Cicero's De Officiis, Book 1, sections 1.11.33-1.13.41. Although, it is well known that Julius Caesar did not often follow these necessities.
Christian theory of the Just War begins with Augustine of Hippo and Thomas Aquinas.
The Just War theory, with some amendments, is still used by
Christians today as a guide to whether or not a war can be justified.
War may be necessary and right, even though it may not be good. In the case of a country that has been invaded by an occupying force, war may be the only way to restore justice. 
Saint Augustine held that, while individuals should not resort immediately to violence, God has given the sword to government for good reason (based upon Romans 13:4). In Contra Faustum Manichaeum book 22 sections 69-76, Augustine argues that Christians, as part of a government, need not be ashamed of protecting peace and punishing wickedness when forced to do so by a government. Augustine asserted that this was a personal, philosophical stance: "What is here required is not a bodily action, but an inward disposition. The sacred seat of virtue is the heart."
Nonetheless, he asserted, peacefulness in the face of a grave wrong that could only be stopped by violence would be a sin. Defense of one's self or others could be a necessity, especially when authorized by a legitimate authority:
They who have waged war in obedience to the divine command, or in conformity with His laws, have represented in their persons the public justice or the wisdom of government, and in this capacity have put to death wicked men; such persons have by no means violated the commandment, "Thou shalt not kill."
While not breaking down the conditions necessary for war to be just, Augustine nonetheless originated the very phrase itself in his work The City of God:
But, say they, the wise man will wage Just Wars. As if he would not all the rather lament the necessity of just wars, if he remembers that he is a man; for if they were not just he would not wage them, and would therefore be delivered from all wars.
J. Mark Mattox writes that, for the individual Christian under the rule of a government engaged in an immoral war, Augustine admonished that Christians, "by divine edict, have no choice but to subject themselves to their political masters and [should] seek to ensure that they execute their war-fighting duty as justly as possible."
In Summa Theologica Aquinas asserted that it is not always a sin to wage war and set out criteria for a just war. According to Aquinas, three requirements must be met: First, the war must be waged upon the command of a rightful sovereign. Second, the war needs to be waged for just cause, on account of some wrong the attacked have committed. Thirdly, warriors must have the right intent, namely to promote good and to avoid evil. Aquinas came to the conclusion that a just war could be offensive and that injustice should not be tolerated so as to avoid war. Nevertheless, Aquinas argued that violence must only be used as a last resort. On the battlefield, violence was only justified to the extent it was necessary. Soldiers needed to avoid cruelty and a just war was limited by the conduct of just combatants. Aquinas argued that it was only in the pursuit of justice, that the good intention of a moral act could justify negative consequences, including the killing of the innocent during a war.
The School of Salamanca expanded on Thomistic understanding of natural law and just war. It stated that war is one of the worst evils suffered by mankind. The School's adherents reasoned that war should be a last resort, and only then, when necessary to prevent an even greater evil. Diplomatic resolution is always preferable, even for the more powerful party, before a war is started. Examples of "just war" are:
In self-defense, as long as there is a reasonable possibility of success.
Preventive war against a tyrant who is about to attack.
War to punish a guilty enemy.
War is not legitimate or illegitimate simply based on its original motivation: it must comply with a series of additional requirements:
It is necessary that the response be commensurate with the evil; use of more violence than is strictly necessary would constitute an unjust war.
Governing authorities declare war, but their decision is not sufficient cause to begin a war. If the people oppose a war, then it is illegitimate. The people have a right to depose a government that is waging or is about to wage an unjust war.
Once war has begun, there remain moral limits to action. For example, one may not attack innocents or kill hostages.
It is obligatory to take advantage of all options for dialogue and negotiations before undertaking a war; war is only legitimate as a last resort.
Under this doctrine expansionist wars, wars of pillage, wars to convert infidels or pagans, and wars for glory are all inherently unjust.
First World War
In the early part of the First World War, a group of theologians in Germany published a manifesto seeking to justify the actions of the German government. At the British government's request Randall Davidson, Archbishop of Canterbury, took the lead in collaborating with a large number of other religious leaders, including some with whom he had differed in the past, to write a rebuttal of the Germans' contentions. Both German and British theologians based themselves on the Just War theory, each group seeking to prove that it applied to the war waged by their own side.
If this responsibility justifies the possession of sufficient means to exercise this right to defense, States still have the obligation to do everything possible "to ensure that the conditions of peace exist, not only within their own territory but throughout the world". It is important to remember that "it is one thing to wage a war of self-defense; it is quite another to seek to impose domination on another nation. The possession of war potential does not justify the use of force for political or military objectives. Nor does the mere fact that war has unfortunately broken out mean that all is fair between the warring parties".
The Charter of the United Nations intends to preserve future generations from war with a prohibition against force to resolve disputes between States. Like most philosophy, it permits legitimate defense and measures to maintain peace. In every case, the charter requires that self-defense must respect the traditional limits of necessity and proportionality.
Therefore, engaging in a preventive war without clear proof that an attack is imminent cannot fail to raise serious moral and juridical questions. International legitimacy for the use of armed force, on the basis of rigorous assessment and with well-founded motivations, can only be given by the decision of a competent body that identifies specific situations as threats to peace and authorizes an intrusion into the sphere of autonomy usually reserved to a State.
Peace, as taught by Sacred Scripture and the experience of men itself, is more than just the absence of war. And the Christian is aware that on earth a human society that is completely and always peaceful is, unfortunately, an utopia and that the ideologies which present it as easily attainable only nourish vain hopes. The cause of peace will not go forward by denying the possibility and the obligation to defend it.
Russian Orthodox Church and the Just War
The War and Peace section in the Basis of the Social Concept of the Russian Orthodox Church is crucial for understanding the Russian Orthodox Church's attitude towards war. The document offers criteria of distinguishing between an aggressive war, which is unacceptable, and a justified war, attributing the highest moral and sacred value of military acts of bravery to a true believer who participates in a "justified" war.
Additionally, the document considers the just war criteria as developed in Western Christianity eligible for Russian Orthodoxy, so the "justified war" idea in Western theology is applicable to the Russian Orthodox Church too.
In the same document is stated to wars have accompanied human history since the fall and, according to the Gospel, will continue to accompany it. While recognizing war as evil, the Russian Orthodox Church does not prohibit her members from participating in hostilities if at stake is the security of their neighbors and the restoration of trampled justice. Then war is considered to be necessary, though undesirable, but means.
Also, it is stated to Orthodoxy has had profound respect for soldiers who gave their lives to protect the life and security of their neighbors.
Just war theorists combine 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 in 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". Although the criticism can be made that the application of just war theory is relativistic, one of the fundamental bases of the tradition is the Ethic of Reciprocity, particularly when it comes to in bello considerations of deportment during battle. If one set of combatants promise to treat their enemies with a modicum of restraint and respect, then the hope is that other sets of combatants will do similarly in reciprocation (a concept not unrelated to the considerations of Game Theory).
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.
Soviet leader Vladimir Lenin defined only three types of just war, all of which share the central trait of being revolutionary in character. In simple terms: "To the Russian workers has fallen the honor and the good fortune of being the first to start the revolution--the great and only legitimate and just war, the war of the oppressed against the oppressors.", with these two opposing categories being defined in terms of class, as is typical in the left. In that manner, Lenin shunned the more common interpretation of a defensive war as a just one--often summarized as "who fired the first shot?"--precisely because it didn't take in consideration the class factor. Which side initiated aggressions or had a grievance or any other commonly considered factor of jus ad bellum mattered not at all, he claimed; if one side was being oppressed by the other, the war against the oppressor would always be, by definition, a defensive war anyway. Any war lacking this duality of oppressed and oppressor was, in contradistinction, always a reactionary, unjust war, in which the oppressed effectively fight in order to protect their own oppressors:
"But picture to yourselves a slave-owner who owned 100 slaves warring against a slave-owner who owned 200 slaves for a more "just" distribution of slaves. Clearly, the application of the term "defensive" war, or war "for the defense of the fatherland" in such a case would be historically false, and in practice would be sheer deception of the common people, of philistines, of ignorant people, by the astute slaveowners. Precisely in this way are the present-day imperialist bourgeoisie deceiving the peoples by means of "national ideology" and the term "defense of the fatherland" in the present war between slave-owners for fortifying and strengthening slavery."
Anarcho-capitalist scholar Murray Rothbard stated: "a just war exists when a people tries to ward off the threat of coercive domination by another people, or to overthrow an already-existing domination. A war is unjust, on the other hand, when a people try to impose domination on another people or try to retain an already existing coercive rule over them."
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 that 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 AD 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 that drew whatever ethical coloring it had from the intentions of the perpetrators.
Just War Theory has two sets of criteria, the first establishing jus ad bellum (the right to go to war), and 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 overcome 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. "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".
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.
In modern terms, just war is waged in terms of self-defense, or in defense of another (with sufficient evidence).
Jus in bello
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.
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).
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, 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 labeled terrorists. This 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 labeling and public opinion, that people use jus post bellum to justify the pursuit of labeled terrorist for the safety of the government's state in a modern context. The actual fault lies with the aggressor, so by being the aggressor they forfeit their rights for honorable treatment by their actions. This is the theory used to justify the actions taken by anyone fighting in a war to treat prisoners outside of war. Actions after a conflict can be warranted by actions observed during war, meaning that there can be justification to meet violence with violence even after war. Orend, who was one of the theorists mentioned earlier, 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.
Militarism - Militarism is the belief that war is not inherently bad but can be a beneficial aspect of society.
Realism - The core proposition of realism is a skepticism as to whether moral concepts such as justice can be applied to the conduct of international affairs. Proponents of realism believe that moral concepts should never prescribe, nor circumscribe, a state's behavior. Instead, a state should place an emphasis on state security and self-interest. One form of realism - descriptive realism - proposes that states cannot act morally, while another form - prescriptive realism - argues that the motivating factor for a state is self-interest. Just wars that violate Just Wars principles effectively constitute a branch of realism.
Revolution and civil war - Just war theory states that a just war must have just authority. To the extent that this is interpreted as a legitimate government, this leaves little room for revolutionary war or civil war, in which an illegitimate entity may declare war for reasons that fit the remaining criteria of just war theory. This is less of a problem if the "just authority" is widely interpreted as "the will of the people" or similar. Article 3 of the 1949 Geneva Conventions side-steps this issue by stating that if one of the parties to a civil war is a High Contracting Party (in practice, the state recognized by the international community), both Parties to the conflict are bound "as a minimum, the following [humanitarian] provisions". Article 4 of the Third Geneva Convention also makes clear that the treatment of prisoners of war is binding on both parties even when captured soldiers have an "allegiance to a government or an authority not recognized by the Detaining Power".
Absolutism - Absolutism holds that there are various ethical rules that are absolute. Breaking such moral rules is never legitimate and therefore is always unjustifiable.
Right of self-defense - The theory of self-defense based on rational self-interest maintains that the use of retaliatory force is justified against repressive nations that break the non-aggression principle. In addition, if a free country is itself subject to foreign aggression, it is morally imperative for that nation to defend itself and its citizens by whatever means necessary. Thus, any means to achieve a swift and complete victory over the enemy is imperative. This view is prominently held by Objectivists.
Pacifism - Pacifism is the belief that war of any kind is morally unacceptable and/or pragmatically not worth the cost. Pacifists extend humanitarian concern not just to enemy civilians but also to combatants, especially conscripts. For example, Ben Salmon believed all war to be unjust. He was sentenced to death during World War I (later commuted to 25 years hard labor) for desertion and spreading propaganda.
Consequentialism - The moral theory most frequently summarized in the words "the end justifies the means", which tends to support the just war theory (unless the just war causes less beneficial means to become necessary, which further requires worst actions for self-defense with bad consequences).
List of just war theorists
These theorists either approve of war as retaliation, or of war as a last resort.
^William Warde Fowler, The Roman Festivals of the Period of the Republic (London 1925), pp. 33ff.; M. Kaser, Das altroemische Ius (Goettingen 1949), pp. 22ff; P. Catalano, Linee del sistema sovrannazionale romano (Torino 1965), pp. 14ff.; W. V. Harris, War and imperialism in Republican Rome, 327-70 B.C. (Oxford 1979), pp. 161 ff.
Brough, Michael W., John W. Lango, Harry van der Linden, eds., Rethinking the Just War Tradition (Albany, NY: SUNY Press, 2007). Discusses the contemporary relevance of just war theory. Offers an annotated bibliography of current writings on just war theory.
Khawaja, Irfan. Review of Larry May, War Crimes and Just War, in Democratiya 10, (), an extended critique of just war theory.
MacDonald, David Roberts. Padre E. C. Crosse and 'the Devonshire Epitaph': The Astonishing Story of One Man at the Battle of the Somme (with Antecedents to Today's 'Just War' Dialogue), 2007 Cloverdale Books, South Bend. ISBN978-1-929569-45-8
McMahan, Jeff. "Just Cause for War," Ethics and International Affairs, 2005.