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The just war theory (Latin: bellum iustum) is a doctrine, also referred to as a tradition, of military ethics which is studied by military leaders, theologians, ethicists and policy makers. The purpose of the doctrine is to ensure that a war is morally justifiable through a series of criteria, all of which must be met for a war to be considered just. The criteria are split into two groups: jus ad bellum ("right to go to war") and jus in bello ("right conduct in war"). The first group of criteria concerns the morality of going to war, and the second group of criteria concerns the moral conduct within war. There have been calls for the inclusion of a third category of just war theory (jus post bellum) dealing with the morality of post-war settlement and reconstruction. The just war theory postulates the belief that war, while it is 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 the just war theory may either be inclined to a stricter pacifist standard (proposing that there has never been nor can there ever be a justifiable basis for war) or they may be inclined toward a more permissive nationalist standard (proposing that a war need only to serve a nation's interests to be justifiable). In many cases, philosophers state that individuals do not need to be plagued by a guilty conscience if they are required to fight. A few philosophers ennoble the virtues of the soldier while they also declare 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. Egyptian ethics of war usually centered on three main ideas, these including the cosmological role of Egypt, the pharaoh as a divine office and executor of the will of the gods, and the superiority of the Egyptian state and population over all other states and peoples. Egyptian political theology held that the pharaoh had the exclusive legitimacy in justly initiating a war, usually claimed to carry out the will of the gods. Senusret I in the Twelfth Dynasty claimed that "I was nursed to be a conqueror...his [Atum] son and his protector, he gave me to conquer what he conquered." Later pharaohs also considered their sonship of the god Amun-Re as granting them absolute ability to declare war on the deity's behalf. Pharaohs often visited temples prior to initiating campaigns, where the pharaoh was believed to receive their commands of war from the deities. So, for example, Kamose claimed that "I went north because I was strong (enough) to attack the Asiatics through the command of Amon, the just of counsels." A stela erected by Thutmose III at the Temple of Amun at Karnak "provides an unequivocal statement of the pharaoh's divine mandate to wage war on his enemies." As the period of the New Kingdom progressed and Egypt heightened its territorial ambition, so did the invocation of just war aid the justification of these efforts. The universal principle of Maat, signifying order and justice, was central to the Egyptian notion of just war and its ability to guarantee Egypt virtually no limits on what it could take, do, or use to guarantee the ambitions of the state.
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 Indian Hindu epic, 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.
The notion of just war in Europe originates and is developed first in ancient Greece, and then in the Roman Empire.
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 argues that the cultivation of a military is necessary and good for the purpose of self-defense, not for conquering: "The proper object of practising military training is not in order that men may enslave those who do not deserve slavery, but in order that first they may themselves avoid becoming enslaved to others" (Politics, Book 7).
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 around the time of Augustine of Hippo 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, "In terms of the traditional notion of jus ad bellum (justice of war, that is, the circumstances in which wars can be justly fought), war is a coping mechanism for righteous sovereigns who would ensure that their violent international encounters are minimal, a reflection of the Divine Will to the greatest extent possible, and always justified. In terms of the traditional notion of jus in bello (justice in war, or the moral considerations which ought to constrain the use of violence in war), war is a coping mechanism for righteous combatants who, by divine edict, have no choice but to subject themselves to their political masters and seek to ensure that they execute their war-fighting duty as justly as possible."
The just war theory by Thomas Aquinas has had a lasting impact on later generations of thinkers and was part of an emerging consensus in Medieval Europe on just war. In the 13th century Aquinas reflected in detail on peace and war. Aquinas was a Dominican friar and contemplated the teachings of the Bible on peace and war in combination with ideas from Aristotle, Plato, Saint Augustine and other philosophers whose writings are part of the Western canon. Aquinas' views on war drew heavily on the Decretum Gratiani, a book the Italian monk Gratian had compiled with passages from the Bible. After its publication in the 12th century, the Decretum Gratiani had been republished with commentary from Pope Innocent IV and the Dominican friar Raymond of Penafort. Other significant influences on Aquinas just war theory were Alexander of Hales and Henry of Segusio.
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:
War is not legitimate or illegitimate simply based on its original motivation: it must comply with a series of additional requirements:
Under this doctrine expansionist wars, wars of pillage, wars to convert infidels or pagans, and wars for glory are all inherently unjust.
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.
The just war doctrine of the Catholic Church found in the 1992 Catechism of the Catholic Church, in paragraph 2309, lists four strict conditions for "legitimate defense by military force":
The Compendium of the Social Doctrine of the Church elaborates on the Just War Doctrine in paragraphs 500 to 501:
Pope John Paul II in an address to a group of soldiers said the following:
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.
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 theory in Western theology is also applicable to the Russian Orthodox Church.
In the same document, it is stated that wars have accompanied human history since the fall of man; according to the gospel, they will continue to accompany it. While recognizing war as evil, the Russian Orthodox Church does not prohibit her members from participating in hostilities if there is the security of their neighbors and the restoration of trampled justice at stake. War is considered to be necessary but undesirable. It is also stated that the Russian Orthodox Church has had profound respect for soldiers who gave their lives to protect the life and security of their neighbors.
The just war theory by the Medieval Christian philosopher Thomas Aquinas was developed further by legal scholars in the context of international law. Cardinal Cajetan, the jurist Francisco de Vitoria, the two Jesuit priests Luis de Molina and Francisco Suárez, as well as the humanist Hugo Grotius and the lawyer Luigi Taparelli were most influential in the formation of a just war tradition. This just war tradition was well established by the 19th century and found its practical application in the Hague Peace Conferences and the founding of the League of Nations in 1920. After the United States Congress declared war on Germany in 1917, Cardinal James Gibbons issued a letter that all Catholics were to support the war because "Our Lord Jesus Christ does not stand for peace at any price... If by Pacifism is meant the teaching that the use of force is never justifiable, then, however well meant, it is mistaken, and it is hurtful to the life of our country"
Armed conflicts such as the Spanish Civil War, World War II and the Cold War were, as a matter of course, judged according to the norms that Aquinas' just war theory had established by philosophers such as Jacques Maritain, Elizabeth Anscombe and John Finnis.
The first work dedicated specifically to just war was the 15th-century sermon De bellis justis of Stanisław of Skarbimierz (1360–1431), who justified war by the Kingdom of Poland against the Teutonic Knights. Francisco de Vitoria criticized the conquest of America by the Kingdom of Spain on the basis of just war theory. With Alberico Gentili and Hugo Grotius just war theory was replaced by international law theory, codified as a set of rules, which today still encompass the points commonly debated, with some modifications.
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 that "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."
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 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).
Main article: Jus ad bellum
In modern terms, just war is waged in terms of self-defense, or in defense of another (with sufficient evidence).
Once war has begun, just war theory (jus in bello) also directs how combatants are to act or should act:
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: