What is just war theory? A beginner’s guide to the ethics of war

Just war theory is the name for a long tradition of argument about two questions. When, if ever, is it right to go to war? And once a war has begun, what may and may not be done in fighting it? The tradition sits between two simpler positions. Pacifism says war is never justified. Realism, in its bluntest form, says morality has nothing to say about war at all, only interest and power. Just war thinkers reject both: war can sometimes be justified, and it is always governed by moral limits.

This guide sets out the standard criteria, where they came from, and the arguments that are still very much alive. The card on war and political violence puts the central question in one line: can killing be permitted when it would otherwise be murder?

Two sets of questions: jus ad bellum and jus in bello

The tradition divides its rules under two Latin headings.

Jus ad bellum, the justice of going to war, usually lists these conditions:

• Just cause. Classically, self-defence against aggression, defence of others, or, more controversially, punishing a grave wrong.
• Legitimate authority. War must be declared by those with the standing to do so, not by private individuals or factions.
• Right intention. The aim must be the just cause itself, not revenge, plunder or glory.
• Last resort. Peaceful means must have been tried, or be plainly hopeless.
• Reasonable chance of success. A good cause does not justify sending people to die in a hopeless war.
• Proportionality. The good to be achieved must outweigh the harm the war will cause.

Jus in bello, justice in the conduct of war, has fewer rules, but they bite harder:

• Discrimination, or distinction. Combatants may be targeted; civilians may not be deliberately attacked.
• Proportionality. Even an attack on a legitimate target must not cause harm to civilians out of proportion to its military value.
• Necessity. Force beyond what the military aim requires is ruled out.

Many recent writers add a third heading, jus post bellum, justice after war: how peace is made, what is owed in reparation, and how the defeated are treated.

Where the criteria came from

No single author invented the list. Cicero, in On Duties, argued that war should be waged only so that people may live in peace without injury, and that it must be properly declared. Augustine, writing as a bishop in a collapsing empire, never wrote a treatise on war, but scattered remarks in his letters and in works such as Against Faustus shaped everything after him. A war may be just, he suggested, if it rights a wrong, and what is evil in war is not chiefly that people die, who will die in any case, but the love of violence, cruelty and the lust for domination.

Thomas Aquinas gathered these threads in the Summa Theologiae (II-II, question 40), naming three conditions: the authority of the ruler, a just cause, and right intention. Elsewhere in the same work, discussing killing in self-defence, he sets out the reasoning later called the doctrine of double effect: one act can have two effects, one intended and the other only foreseen. That doctrine still shapes how many people think about civilian deaths in war.

The Islamic tradition developed its own law of war, siyar, with the eighth-century work of Muḥammad al-Shaybānī as a landmark. Early reports attribute to the first caliph, Abū Bakr, instructions to his troops not to kill women, children or the old and not to cut down fruit trees, and the jurists debated in detail who counted as a combatant. Indian epic and legal literature has rules of righteous combat too, for instance against striking an opponent who is unarmed or has surrendered.

In the sixteenth century Francisco de Vitoria, lecturing at Salamanca on the Spanish conquest of the Americas, argued that difference of religion is not a just cause of war, and that a soldier convinced a war is unjust should not fight in it. His lectures are in the Book corner as On the American Indians. Hugo Grotius's On the Law of War and Peace (1625) then carried the tradition into the language of the law of nations, which is the route by which many of these ideas entered modern international law.

The modern debate

In the twentieth century the theory had to face aerial bombing and nuclear weapons. In 1956 Elizabeth Anscombe opposed Oxford's award of an honorary degree to Harry Truman, arguing that the deliberate killing of civilians at Hiroshima and Nagasaki was murder, whatever its supposed benefits. Her pamphlet, Mr Truman's Degree, remains one of the clearest defences of the principle of discrimination as an absolute rule.

Michael Walzer's Just and Unjust Wars (1977) revived the subject for a general audience. One of its central claims is the moral equality of soldiers: the justice of the war is the leaders' responsibility, while soldiers on both sides are equally permitted to fight, provided they obey the rules of conduct. This is roughly how the laws of war treat combatants, and it is why a captured soldier is a prisoner, not a criminal.

The most important challenge comes from Jeff McMahan, whose Killing in War (2009) argues that the two sets of rules cannot be kept apart. A soldier fighting for an unjust cause, he says, is like an attacker in a street fight: the defender may resist, but the attacker gains no right to kill the defender, however correctly he fights. McMahan does not conclude that such soldiers should be prosecuted, since they often act under duress or in ignorance and may be excused. But he denies that they are morally equal. The argument is running on the site now, in Choi Byeong-hun's motion about conscripts, written by someone who trained them.

Questions to take further

Just war theory is less a finished doctrine than a set of questions that keep returning in new forms. Some of the live ones:

• Can the principle of discrimination survive wars fought from the air, by remote weapons, or against fighters who wear no uniform?
• Is humanitarian intervention, war to stop atrocities in another country, a just cause, or does it make war too easy to begin?
• Does the moral equality of soldiers protect conscripts, or merely excuse them?
• Is pacifism, which the tradition was built to answer, really refuted, or only outnumbered?

If you are new to the subject, start with the card above, read Aquinas's short question 40, and then the opening chapters of Walzer. You will probably find, as most readers do, that the criteria seem obvious until you try to apply them to a real case.