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Can the deliberate killing involved in war ever be morally right, or is all war wrong? Christianity has given three broad answers — the just war tradition, which holds that war can be justified under strict conditions; pacifism, which holds that violence is always wrong; and a realist recognition that politics sometimes forces tragic choices. This lesson sets out the just war criteria under their three headings (jus ad bellum, jus in bello and jus post bellum), traces them from Augustine and Aquinas to the modern theorist Michael Walzer, examines the varieties of pacifism (absolute, contingent and nuclear) and their leading critics, and then applies the framework to the hardest modern case, weapons of mass destruction. Although war is treated in the Christianity component of AQA 7062, the underlying reasoning is the ethical machinery of natural law, deontology and consequentialism, and a strong answer will connect the two.
The tradition holds that war, though always an evil, can be a necessary and therefore justifiable evil, provided it is entered and fought within moral limits. St Augustine of Hippo (354–430), writing as the Roman Empire crumbled, is usually called its father. Faced with the question of how a Christian committed to "love your enemies" could serve a state that must sometimes fight, Augustine argued that the use of force could itself be an act of love of neighbour: to defend the innocent from a violent aggressor, and even to restrain the aggressor for their own and others' good, can be what charity requires. War, for Augustine, must be authorised by legitimate authority and waged with right intention — the restoration of a just peace, not hatred, cruelty or the lust for domination, which he regarded as the true evils of war. It is worth noticing the tragic character of Augustine's view: he does not glorify war or pretend it is good, but presents it as a grievous necessity in a fallen world, to be entered with sorrow rather than enthusiasm and always ordered towards the peace it is meant to restore. This is a different temper from the crusading zeal it is sometimes confused with, and it explains why the tradition that descends from Augustine treats every just war as a regrettable exception rather than a cause for celebration.
St Thomas Aquinas (1225–1274), in the Summa Theologica (II-II, Q.40), distilled the tradition into three conditions for a just war: legitimate authority (auctoritas principis) — war may be declared only by a sovereign public authority, not by private persons; just cause (causa justa) — those attacked must deserve it on account of some fault, typically aggression or grave wrongdoing; and right intention (intentio recta) — the aim must be the advancement of good and the avoidance of evil, the securing of peace, not conquest or revenge. Later scholastics and jurists (Vitoria, Suárez, Grotius) expanded these into the fuller list of jus ad bellum criteria below, and the principles governing conduct in war were drawn out as jus in bello.
Key term: Jus ad bellum — the set of conditions that must be met for the decision to go to war to be just (just cause, legitimate authority, right intention, last resort, proportionality, reasonable chance of success).
These criteria are cumulative: a war is just only if it satisfies all of them, which sets a deliberately high bar. Applying them to a concrete case sharpens the point. The 2003 invasion of Iraq can be interrogated criterion by criterion: was there a just cause (the claimed weapons of mass destruction were never found)? Was it a genuine last resort (weapons inspections were still ongoing)? Did it have legitimate authority (it lacked explicit UN Security Council authorisation)? Was the intention truly the securing of peace, and was there a realistic plan for jus post bellum? On most readings the war fails several criteria at once — which is exactly why so many ethicists and church leaders opposed it, and a model of how the framework is meant to be used. Yet this also exposes the tradition's great weakness, repeatedly seen in practice: manipulability. Almost every aggressor in history has claimed self-defence, just cause and right intention, so the criteria can degenerate into a rhetorical checklist ticked off to dignify a war already decided upon. This is itself a serious evaluation point: a framework that the powerful can bend to license what they intended anyway may do less restraining work than it promises — though a defender replies that the criteria at least give critics a shared, public vocabulary in which to expose an unjust war, as the Iraq case shows.
Key term: Jus in bello — the rules governing how a war is fought, chiefly discrimination (non-combatant immunity) and proportionality in the means used.
Even a justly begun war must be justly fought, and jus in bello binds both sides regardless of who is in the right. Its two central principles are discrimination (non-combatant immunity) — combatants must distinguish military targets from civilians, and the deliberate killing of civilians is always prohibited — and proportionality of means — the force used must be proportionate to the military objective, so that excessive or indiscriminate violence (the carpet-bombing of cities, the use of poison gas) is forbidden even in a just cause. The tradition also rules out certain intrinsically evil means (the targeting of hospitals; torture; the murder of prisoners) and requires the humane treatment of prisoners of war. Much of this is now codified in international humanitarian law, above all the Geneva Conventions (1949) and their Additional Protocols (1977). The independence of jus in bello from jus ad bellum is important: a soldier fighting in an unjust war can still fight cleanly (observing discrimination and proportionality), and a soldier in a just war can commit war crimes — the two evaluations come apart, a point Walzer makes central.
A third heading, jus post bellum, has been increasingly recognised: the obligations that arise once the fighting stops. A just settlement must be proportionate, not vindictive — the Treaty of Versailles (1919), whose punitive reparations are widely thought to have fed German grievance and the rise of Nazism, is the standard cautionary example. The victor has duties of reconstruction (restoring infrastructure and governance), of accountability (the prosecution of war crimes, from the Nuremberg Trials of 1945–46 to the International Criminal Court established in 2002), and of reconciliation (truth commissions and the long work of healing). The catastrophic aftermath of the 2003 Iraq war — where the failure to plan for the peace arguably caused more suffering than the invasion itself — is frequently cited to show that a war can be lost, morally, after it is won militarily, and that jus post bellum is not an optional appendix but integral to the justice of the whole enterprise.
Before turning to the modern theory, it is worth distinguishing the just war from the holy war (or crusade), since the two are easily confused and the contrast is examinable. A just war is fought for a political cause — defence, the righting of a wrong — under limits that bind both sides and protect the innocent. A holy war is fought for a religious cause, at what is claimed to be God's direct command, often with the promise of spiritual reward (the medieval Crusades offered indulgences) and frequently without the restraints of discrimination and proportionality, since the enemy is cast as the enemy of God. The just war tradition, properly understood, is in fact a restraint on the holy-war impulse: by insisting on legitimate political authority, a this-worldly just cause and non-combatant immunity, it refuses the idea that religious zeal can license unlimited violence. Christian theologians today overwhelmingly repudiate the crusade model; recognising the difference helps a candidate avoid the error of treating "religious war" and "just war" as the same thing, and shows why the appeal to God's command is, if anything, more dangerous than the secular case for war it is sometimes thought to sanctify, since a war believed to be willed by God resists the very limits — proportionality, the protection of the innocent, the openness to a negotiated peace — that make a just war just.
The most influential modern statement is Michael Walzer's Just and Unjust Wars (1977), written in the shadow of Vietnam. Walzer reconstructs just war theory on secular, rights-based foundations: the "legalist paradigm" treats states as having rights to political sovereignty and territorial integrity analogous to individual rights, so that aggression — the violation of those rights — is the paradigm crime that justifies a defensive or punitive war. Two of his contributions are especially examinable. First, he insists on the independence of the two judgements (jus ad bellum and jus in bello): the justice of a war and the justice of its conduct are logically separate, which is why we can condemn the conduct of soldiers fighting in a just cause and respect the clean fighting of soldiers serving an unjust one. Second, he introduces the idea of the "supreme emergency": in a situation of imminent and extreme catastrophe — his example is Britain in 1940–41 facing Nazi conquest — the ordinary rule of non-combatant immunity might, exceptionally and tragically, be overridden, while remaining a genuine wrong for which there is no clean conscience. Critics object that the supreme-emergency exception threatens to swallow the rule, since any belligerent can claim its survival is at stake; defenders reply that Walzer hedges it with stringent conditions (the danger must be both imminent and of an extreme, civilisation-threatening kind) and pointedly refuses to call the overriding "right" — it remains a wrong, a stain, for which the agent bears guilt — which preserves the sense of tragedy that distinguishes his view from a simple consequentialism that would calmly tot up the lives saved. Walzer's framework is valuable for examination precisely because it is not tidy: it holds together a robust defence of non-combatant immunity with a frank acknowledgement that there may be extremities in which moral life offers no clean choice, only a choice between guilt and catastrophe. A candidate who can deploy the independence thesis (to separate the justice of a war from the justice of its conduct) and the supreme-emergency idea (to discuss area bombing in 1940–45 without either excusing or simplistically condemning it) demonstrates command of the modern debate well beyond the medieval criteria.
Key term: Pacifism — the conviction that war and the use of lethal violence are morally wrong; in its absolute form, wrong in all circumstances; in contingent (selective) form, opposed to particular wars; in nuclear form, opposed specifically to nuclear (or all WMD) warfare.
Absolute pacifism holds that violence is wrong without exception — that one may never take human life, whatever the provocation or the cost of refusing. Christian pacifism grounds this in the teaching and example of Jesus. The Sermon on the Mount blesses the peacemakers (Matthew 5:9), sets Jesus's teaching against retaliation — "you have heard that it was said, 'An eye for an eye'... but I say to you, do not resist an evildoer" (Matthew 5:38–39) — and commands "love your enemies and pray for those who persecute you" (Matthew 5:44). At his arrest Jesus tells the disciple who draws a sword to put it away, "for all who take the sword will perish by the sword" (Matthew 26:52). The early Church was largely pacifist for its first centuries, with writers such as Tertullian (c. 155–220) and Origen (c. 185–254) opposing Christian participation in killing; the shift towards acceptance of military service is conventionally associated with the conversion of the Emperor Constantine and the Church's new entanglement with the power of the state in the fourth century — the very moment Augustine's just war theology was taking shape. This historical narrative is itself an argument the pacifist deploys: that just war thinking is less a faithful development of the gospel than an accommodation made when Christianity moved from a persecuted minority to the religion of empire, so that the burden of proof lies with those who would qualify Jesus's plain teaching of non-violence rather than with those who take it at face value. The clearest institutional heir is the Religious Society of Friends (Quakers), whose Peace Testimony (1660) declared that they "utterly deny all outward wars and strife and fightings with outward weapons, for any end or under any pretence whatsoever," grounded in the conviction that there is "that of God in everyone," so that to kill is to assault the divine image in another. In the twentieth century Stanley Hauerwas (b. 1940) reframed Christian pacifism as the Church's vocation to be a community of peace — not a policy it lobbies the state to adopt, but a distinctive form of common life that bears witness to the possibility of non-violence and refuses to underwrite the violence of the nations.
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