The Ethics and Legitimacy of War: The Just War Theory

War is one of the oldest features of human politics, yet societies have never been comfortable treating it as a free-for-all. From ancient times, thinkers have asked a difficult question: can killing on a massive scale ever be morally right, and if so, under what conditions? The Just War theory is the most influential attempt to answer that question. It rejects two extreme positions – that war is always wrong (pacifism) and that war follows no moral rules at all (realism) – and instead argues that war can sometimes be justified, but only within strict ethical limits. The modern version of this debate owes a great deal to the political philosopher Michael Walzer, whose 1977 book Just and Unjust Wars reshaped how scholars, soldiers, and citizens think about the morality of armed conflict.

What is Just War theory? 🔗

Just War theory is a tradition of ethical and legal thought that sets out the conditions under which going to war can be justified and the rules that must be followed once fighting begins. It is not a single rulebook handed down by one author. It grew over centuries through the writings of thinkers like Augustine, Thomas Aquinas, and Hugo Grotius, and it continues to shape international humanitarian law today.

The core idea is simple but powerful. War causes enormous suffering, so the burden of proof always lies on those who want to start one. There is a strong presumption against violence, and that presumption can only be overcome by serious moral reasons. The theory exists not to make war easier, but to make it rarer and, when it does happen, less destructive.

The two pillars: jus ad bellum and jus in bello 🔗

Just War theory traditionally divides into two distinct sets of principles. Jus ad bellum concerns the justice of going to war – the conditions a state must meet before it can legitimately resort to force. Jus in bello concerns justice within war – the rules governing how soldiers must conduct themselves once fighting has begun. A third category, jus post bellum, has gained attention more recently and deals with justice after war, including how peace settlements and reconstruction should be handled.

One of Walzer’s central claims is that these categories are logically independent of each other. A state can fight a just war (a war of defence) but fight it unjustly (by deliberately bombing civilians). Equally, a soldier on the aggressor’s side can still fight within the rules. Walzer separated jus ad bellum from jus in bello, arguing that an ordinary soldier may legally participate in combat and receive immunity for killing enemy soldiers, even if the war their nation started was an act of illegal aggression. This separation lies at the heart of the theory’s structure.

Michael Walzer and the legalist paradigm 🔗

Walzer’s biggest contribution was to ground Just War theory in the rights of states and the individuals who live within them. He aimed to write a book of practical morality that captured how ordinary citizens, not just lawyers, actually argue about the rights and wrongs of war.

To do this, he built what he called the legalist paradigm. This paradigm rests on a “domestic analogy” – the idea that the international order can be understood by comparing it to the order among individuals within a single society. Just as citizens have rights that others must respect, states are treated as members of an international society with their own rights and duties. When one person attacks another, we call it a crime. When one state attacks another, the analogy suggests, we should treat it the same way.

The six propositions of the theory of aggression 🔗

Walzer summarised the legalist paradigm as a set of clear propositions that together form what he called the theory of aggression. Walzer organised the theory into six propositions, which can be paraphrased as follows:

1. An international society of independent states exists. States, not individuals, are the members of this society, and the default expectation is that states do not interfere in each other’s internal affairs.

2. States have rights of territorial integrity and political sovereignty. These rights ultimately derive from the rights of the individuals who make up the political community.

3. Aggression is a crime. Any use of force, or imminent threat of force, by one state against the territorial integrity or political sovereignty of another constitutes criminal aggression.

4. Aggression justifies two responses: a war of self-defence by the victim, and a war of law enforcement by the victim or any other member of international society.

5. Nothing but aggression can justify war. This rules out wars of conquest, expansion, or preventive attack.

6. Once an aggressor has been defeated, it can be punished. Because states are members of a society with rights, they can also be subjects of punishment.

For Walzer, aggression is the only crime that states can commit against other states – everything else, he argued, is essentially a lesser offence. This is why the entire structure of the theory revolves around resisting and punishing aggression.

Why this paradigm matters 🔗

The purpose of the legalist paradigm is to drastically limit the occasions on which the use of military force is considered legitimate. By insisting that only a response to aggression can justify war, the theory creates a strong presumption against violence and eliminates appeasement, conquest, and intervention as acceptable reasons for fighting. The international system is treated as fragile, where every act of aggression threatens not just one state but the security and rights of all states.

This framework also influenced real-world law. Walzer’s definition of aggression closely tracks the United Nations Charter, which prohibits the threat or use of force against the territorial integrity or political independence of any state, while preserving the inherent right of self-defence. The 1974 UN General Assembly definition of aggression shaped his thinking, showing how moral philosophy and international law reinforce each other.

The principles of jus ad bellum 🔗

Beyond the broad theory of aggression, the just war tradition specifies several conditions that must all be satisfied before war can be considered just. These are worth understanding individually.

Just cause: There must be a genuine and serious reason for war, typically self-defence against aggression or the defence of another state under attack. A desire for resources, glory, or revenge does not qualify.

Legitimate authority: War must be declared by a proper authority, traditionally the sovereign state. International law gives only states the rights of national defence and grants combatant rights primarily to the soldiers of states, which is why non-state actors occupy a contested place in the theory.

Right intention: The aim must be to restore a just peace, not to pursue hidden motives such as territorial gain.

Last resort: All reasonable non-violent options – diplomacy, negotiation, economic pressure – must be exhausted before force is used.

Reasonable probability of success: There must be good grounds to believe the war can actually achieve its aims. Mass violence should not be undertaken if it is unlikely to secure the just cause, since pointless bloodshed cannot be justified.

Proportionality: The overall harm expected from the war must not outweigh the good it seeks to achieve.

The problem of pre-emption 🔗

One of the trickiest issues is whether a state must wait to be attacked before defending itself. Walzer argued that strict waiting is too demanding. A state need not wait until aggression has actually begun if it faces a genuine and imminent threat. He famously concluded that Israel’s 1967 pre-emptive strike against the Arab nations was morally legitimate, because the threat met a high standard of seriousness. For Walzer, a “sufficient threat” requires a manifest intent to injure, active preparation that makes the intent a danger, and a situation where waiting greatly increases the risk. This is a narrow exception – it permits pre-emption against an imminent attack but still forbids preventive war against a distant, hypothetical one.

The principles of jus in bello 🔗

Even a war fought for the most just cause can be waged in deeply unjust ways. The rules of conduct in war – what Walzer called the “war convention” – exist to limit the cruelty of fighting itself.

Discrimination and non-combatant immunity 🔗

The most important rule of jus in bello is discrimination: soldiers must distinguish between legitimate targets and the innocent. The principle of discrimination focuses on who counts as a legitimate target of war, while sparing those who pose no threat. Civilians, prisoners of war, the wounded, and others not engaged in fighting are considered non-combatants and are immune from deliberate attack. Walzer defended this principle of non-combatant immunity against both realists, who dismiss morality in war, and utilitarians, who would sacrifice civilians to end a war faster.

Proportionality and prohibited means 🔗

The second rule is proportionality in combat. Proportionality deals with what kind of force is morally permissible in a given military action, requiring forces to use no more violence than is necessary to achieve a legitimate objective. The third rule prohibits weapons and methods considered “evil in themselves” (mala in se), such as mass poisoning or biological weapons, regardless of how effective they might be.

Criticisms and ongoing debates 🔗

Just War theory remains highly contested, and Walzer’s version in particular has drawn sustained criticism. Understanding these debates is essential to grasping why the topic is still so alive in political theory.

The supreme emergency exemption 🔗

Walzer himself created one of the most controversial elements of his theory. He argued that in cases of supreme emergency – when a political community faces an imminent threat of total destruction, such as the danger of a Nazi victory – the normal rules protecting civilians might be temporarily overridden. Critics argue this exception dangerously undermines the very principle of non-combatant immunity that Walzer otherwise defends so firmly.

The challenge from revisionists 🔗

A school of “revisionist” just war theorists, led by philosophers like Jeff McMahan, challenges the traditional view that jus ad bellum and jus in bello are fully independent. Revisionists question whether unjust combatants can fight permissibly at all, arguing that soldiers fighting for an unjust cause cannot be morally equal to those defending against aggression. If your cause is unjust, they ask, how can the killing you do ever be justified? This strikes directly at Walzer’s idea of the “moral equality of soldiers.”

Non-state actors and modern warfare 🔗

The legalist paradigm was built around sovereign states, but much of today’s conflict involves insurgents, terrorist groups, and other non-state actors. Whether the moral equality of soldiers applies in asymmetric conflicts between states and non-state groups remains deeply unsettled. Drone strikes, cyber warfare, and counter-insurgency operations blur the line between combatant and civilian, putting pressure on a framework designed for conventional state-versus-state war.

Why Just War theory still matters 🔗

For students of peace and conflict studies, Just War theory is more than a historical curiosity. It provides the moral vocabulary that diplomats, courts, and international institutions use when they debate military intervention, self-defence, and war crimes. The principles of distinction and proportionality are written directly into the laws of armed conflict that govern conflicts around the world.

The theory’s deepest value lies in its refusal to accept easy answers. It does not say war is always wrong, nor that it is sometimes acceptable without limits. Instead, it forces anyone considering force to justify it against demanding moral standards – and to keep justifying it at every stage of the conflict. In a world where wars continue to be fought and civilians continue to suffer, that insistence on accountability is exactly why the framework endures.

What do you think? Is Walzer’s “supreme emergency” exemption a realistic acknowledgement of moral tragedy, or does it fatally weaken the protection of innocent civilians? And in an age of drones, cyberattacks, and non-state armed groups, can a theory built around sovereign states still guide us toward more ethical conflict?

References

1. https://www.icrc.org/en/law-and-policy
2. https://plato.stanford.edu/entries/war/
3. https://www.hoover.org/research/michael-walzer-just-and-unjust-wars-1977
4. https://www.supersummary.com/just-and-unjust-wars/part-2-summary/
5. https://people.brandeis.edu/~teuber/Walzerchapter4.pdf
6. https://www.un.org/en/about-us/un-charter
7. https://en.wikipedia.org/wiki/Jus_ad_bellum
8. https://iep.utm.edu/justwar/
9. https://www.beyondintractability.org/essay/jus_in_bello
10. https://link.springer.com/article/10.1007/s11406-006-9007-y
11. https://www.amacad.org/publication/daedalus/just-war-theory-conduct-asymmetric-warfare