Just War Theory

In brief

Just war theory occupies the middle ground between two positions it rejects. Against pacifism, it holds that some wars are justified: a state may defend itself and its people against aggression. Against political realism, which treats war as a domain where morality falls silent and only power and interest speak, it holds that war is a human activity like any other, and so subject to moral judgment throughout. The tradition runs from Augustine in the fifth century, through Thomas Aquinas in the thirteenth, to the international lawyers Francisco de Vitoria and Hugo Grotius in the sixteenth and seventeenth, and into the present, revived for a modern audience by Michael Walzer's Just and Unjust Wars (1977).

Its central move is a division of the moral question in two. First, was it right to go to war at all? This is jus ad bellum, the justice of resorting to war, judged by the cause, the authority, the intention, and whether force was proportionate and a genuine last resort. Second, quite separately, is the war being fought by permissible means? This is jus in bello, the justice of conduct in war, governed above all by two rules: discrimination, which forbids the deliberate targeting of civilians, and proportionality, which forbids force out of proportion to the military goal. A war can be just to begin and fought unjustly, or unjust to begin yet fought within the rules. Keeping the two questions apart is the tradition's defining discipline.

The full treatment

The problem it answers

War is the most destructive thing human beings organize, and the temptation is to say one of two simple things about it. The pacifist says all killing in war is murder, so no war is ever permitted. The realist says that between states there is no common judge and no shared morality, so talk of a just war is sentimental cover for the pursuit of interest. Both are clean positions. Both are hard to live by. Most people believe that resisting a genocidal invasion is not merely permitted but obligatory, which the pacifist cannot allow; and most people believe that massacring prisoners or gassing a city is wrong even when it wins, which the strict realist cannot explain. Just war theory is the attempt to hold on to both of those intuitions at once. War is sometimes justified, and it is never a moral free zone. The theory's job is to say precisely when the resort to war is justified and precisely what may be done once it starts.

How it works: the two questions, kept apart

The tradition's architecture is a deliberate separation.

Jus ad bellum, the justice of going to war, is a checklist of conditions that a resort to force must satisfy, and it must satisfy all of them. Just cause: there must be a real wrong to be righted, paradigmatically defense against aggression, not the pursuit of glory, territory, or trade. Legitimate authority: war may be declared only by the proper public authority, not by private persons or factions. Right intention: the aim must be the just cause itself and the peace that follows, not hatred or conquest smuggled in behind it. Last resort: peaceful alternatives must be exhausted or clearly futile. Proportionality: the overall good expected must outweigh the overall harm, so a small wrong does not license a catastrophic war. Reasonable prospect of success: hopeless resistance that only multiplies death is hard to justify.

Jus in bello, the justice of conduct within war, is a separate test applied to how the fighting is done, and it rests on two principles. Discrimination, or the principle of noncombatant immunity: those who are not fighting, civilians, medical staff, prisoners, the wounded, may not be made the direct object of attack. Proportionality in bello: the harm inflicted in a given operation, including foreseeable harm to civilians, must not be excessive in relation to the concrete military advantage sought.

The separation matters because it lets us say two things that ordinary talk runs together. A soldier fighting in an unjust war (say, an invading conscript) can still fight cleanly, and a soldier in a just cause can commit atrocities. Traditional just war theory judges each level on its own terms.

The doctrine of double effect: how noncombatants may be harmed at all

If discrimination forbids targeting civilians, how can any modern bombing be permitted, when civilian deaths are almost certain? The tradition's answer, inherited from Aquinas, is the doctrine of double effect (see the-doctrine-of-double-effect). It distinguishes harms you intend, as ends or as means, from harms you merely foresee as side effects. Bombing a munitions factory and foreseeing that some workers nearby will die is, on this view, different in kind from bombing a residential district in order to terrorize the population, even if the body counts are similar. The first treats civilian deaths as a regretted side effect; the second uses them as the very instrument of victory. The distinction is doing enormous moral work, and it is exactly where critics press hardest, because "foreseen but not intended" can become a formula for excusing almost anything.

Jus post bellum: the justice of the peace

The oldest parts of the tradition said little about what happens after the guns stop. Recent theorists, notably Brian Orend and Gary Bass, have added a third category, jus post bellum, the justice of the settlement. Its concerns are proportionate peace terms rather than vindictive ones, accountability for war crimes without collective punishment of a whole people, and rebuilding rather than plunder. The Carthaginian peace Rome imposed, and the punitive Treaty of Versailles in 1919 (often blamed for helping produce the next war), are the cautionary cases; the reconstruction of Germany and Japan after 1945 is the frequently cited counterexample.

The key modern text: Walzer

Michael Walzer's Just and Unjust Wars, written in the shadow of Vietnam and published in 1977, is the book that carried the tradition into contemporary debate. Walzer argues largely through historical cases rather than abstract axioms: the German invasion of Belgium in 1914, the Allied bombing of German cities, Hiroshima, the Six Day War, My Lai. He defends what he calls the "war convention," the body of rules soldiers actually recognize, and grounds jus ad bellum in a "legalist paradigm" built on the rights of states to political sovereignty and territorial integrity, with aggression defined as the violation of those rights. His most discussed and most contested idea is the "supreme emergency" exception: he argues that when a political community faces an imminent threat of enslavement or extermination, as Britain plausibly did in 1940, the ordinary prohibition on killing civilians may, tragically, be overridden. It is a deliberate crack in the wall he otherwise defends, and it has generated decades of argument.

Lineage

The tradition has a double parentage. Its ethical spine descends from natural law (see natural-law): the idea that there are moral norms accessible to reason and binding on rulers and ruled alike. Augustine of Hippo (354 to 430), reconciling Christian nonviolence with the duties of a Christian Roman official, argued in The City of God (completed 426) that a Christian may wage war, sorrowfully, to restore a wrongfully broken peace. Thomas Aquinas (1225 to 1274), drawing on Aristotle (see aristotle) and on Augustine, gave the canonical formulation in the Summa Theologiae: for a war to be just it requires the authority of the sovereign, a just cause, and a right intention.

In the sixteenth century the Spanish scholastics Francisco de Vitoria (c. 1483 to 1546) and later Francisco Suarez extended the analysis to the conquest of the Americas, asking whether the Spanish had any just cause against peoples who had done them no wrong. Then Hugo Grotius (1583 to 1645), in On the Law of War and Peace (1625), cut the tradition loose from its theological moorings and reframed it as a law of nations, binding among states whether or not they shared a religion. That secular, legal descendant is the direct ancestor of modern international humanitarian law: the Hague Conventions, the Geneva Conventions of 1949, and the United Nations Charter's rules on the use of force. The doctrine of the responsibility to protect (see responsibility-to-protect), adopted at the UN in 2005, is a recent descendant, extending just cause to the defense of populations against their own governments.

The strongest case for it

The strongest case is that just war theory is the only serious position that matches the moral experience of most people while giving that experience discipline. It refuses the two evasions. It will not tell the victim of aggression to submit, and it will not tell the victor that anything goes. It gives commanders and soldiers usable rules, and those rules have been codified into law that has, imperfectly but really, restrained conduct: the protection of prisoners, the immunity of medical units, the prohibition of poison gas and of attacks whose sole purpose is to terrorize. The separation of jus ad bellum from jus in bello is itself a moral achievement, because it lets us condemn a war's launch while still holding every soldier on both sides to the same standard of conduct, which is the only standard an army in the field can actually be trained and prosecuted against. And by insisting that even a just war has limits, the tradition denies the most dangerous idea in politics: that a good enough end can license any means. That denial is what separates it cleanly from the logic of the ticking-time-bomb scenario and from every argument that the emergency justifies the atrocity.

The strongest case against it

The objections come from three directions and none is frivolous.

From political realism (see political-realism): thinkers in the tradition of Thucydides, and in the twentieth century figures associated with realist thought such as Hans Morgenthau and, in a harsher register, Carl Schmitt, argue that between sovereign states there is no shared authority to make moral rules binding, so just war language is either naive or, worse, a weapon. Naming your enemy "unjust" does not restrain war; it inflames it, converting a limited contest of interests into a crusade against evil in which no mercy is owed. On this view the theory's categories are used, in practice, to dress aggression as police work.

From pacifism: absolute pacifists, and figures such as Leo Tolstoy, argue that the theory's conditions are so elastic that no state has ever failed to convince itself its wars were just, so in practice just war theory licenses the very slaughter it pretends to limit. The checklist becomes a ritual of self-justification. The demand for a "reasonable prospect of success" and a "proportionate" cause is a matter of judgment that governments will always resolve in their own favor.

From within moral philosophy, the sharpest recent attack is the revisionist critique, led by Jeff McMahan in Killing in War (2009). McMahan targets the tradition's core assumption, defended by Walzer, of the "moral equality of combatants," the claim that soldiers on both sides have the same right to kill and the same liability to be killed regardless of whether their cause is just. McMahan argues this is incoherent: if your cause is unjust, you are like an armed robber, and the fact that you wear a uniform and were ordered to shoot does not give you a moral right to kill the people defending themselves against you. Liability to be killed, he argues, tracks moral responsibility for an unjust threat, not mere combatant status. If he is right, the tidy separation of ad bellum from in bello collapses, and much of ordinary just war reasoning has to be rebuilt. Defenders of the traditional view, including Walzer and Henry Shue, reply that McMahan's individualized morality is unworkable as a public rule of war: soldiers cannot reliably know whose cause is just, and a law of war that only protected the "justified" side would in practice protect no one and be obeyed by no one.

There is also the perennial problem the pacifist and the realist both exploit: every substantive term, proportionality, last resort, reasonable success, is a judgment call made under pressure by the party that wants to fight.

Where it stands now

Just war theory is, at once, the dominant framework in the ethics of war and a tradition in visible ferment. Its vocabulary is the vocabulary of international law and of public argument: no government goes to war today without claiming just cause, proportionality, and last resort, which is itself evidence of how thoroughly the tradition has won the terms of debate, even where states cynically abuse them. Academically, the field has split. On one side stand the "traditionalists" or conventionalists who defend Walzer's framework and its convention-based, state-centric rules. On the other stand the "revisionists," McMahan foremost among them, who rebuild the theory from the individual's right of self-defense and are willing to follow the argument even where it clashes with existing law. Meanwhile new kinds of conflict strain every category: drone strikes and targeted killing test discrimination and legitimate authority; cyberwar tests what even counts as an armed attack; nuclear weapons (see nuclear-deterrence) pose the problem of a strategy of deterrence that rests on threatening exactly the mass killing of civilians the theory forbids; and asymmetric wars against non-state fighters who deliberately blur the line between combatant and civilian make the principle of discrimination hardest to apply where it matters most. The tradition endures because the underlying problem is permanent: force is sometimes necessary, and force is always terrible, and something must stand between those two facts.

Test yourself

Suppose a war was clearly just to begin, a defense against genocidal invasion, but the only way to win quickly is to bomb a city and kill thousands of civilians who are not fighting. Walzer's "supreme emergency" says that in the last extremity this may be permitted; the doctrine of discrimination says it never is. Before you decide, notice which fact you reached for first: the justice of the cause, or the innocence of the dead. The whole tradition turns on refusing to let the first answer the second.