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Debate

War and Peace: When Is Armed Force Justified?

The UN Charter outlaws wars of aggression, and international law allows a state to defend itself. But what may a people under attack and its allies do – and when do the costs of fighting, in lives and in the risk of nuclear escalation, become too high?

For decades many Europeans regarded war between states as a thing of the past. Russia’s full-scale invasion of Ukraine in February 2022, the Hamas attack on Israel of 7 October 2023 and the war in Gaza that followed, and the civil war in Sudan have brought the old questions back with full force: how a state may defend itself, whether others should supply weapons, what is owed to civilians, when to negotiate, and how to live with nuclear deterrence. Governments are rearming, and international courts are examining allegations of war crimes in several of these conflicts. Philosophy has argued about war for more than two thousand years, in China as in Europe. Today’s debate runs between the just war tradition and its revisionist critics, pacifists, realists and those who hope to replace war with law.

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Positions

The just war tradition

Augustine held that a war can be just when it avenges a wrong, and Thomas Aquinas named three conditions: legitimate authority, a just cause and right intention. Vitoria (1539) insisted that the only just cause is a wrong received and that the deliberate killing of innocents is never permitted; Grotius (1625) argued that law binds even in war. In China the Mohists had condemned wars of conquest while perfecting the defence of besieged cities. Michael Walzer’s Just and Unjust Wars (1977) renewed the tradition after Vietnam: aggression is the paradigmatic crime between states and justifies self-defence and help from others, but how a war is fought must be judged separately – soldiers on both sides are moral equals, and civilians may never be made targets. The UN Charter of 1945 turned this into law, banning force except in self-defence or with the authorisation of the Security Council.

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No equality of combatants

Revisionists such as Jeff McMahan (Killing in War, 2009) argue that the rules for fighting cannot be separated from the justice of the cause. Killing in war is justified by the same principles as killing in self-defence between individuals: only those who are liable, because they are responsible for an unjust threat, may be attacked. Soldiers fighting for an unjust cause are therefore not the moral equals of those resisting them – even if the law, for practical reasons, should continue to treat both sides alike. Cécile Fabre (Cosmopolitan War, 2012) and others ground the ethics of war in the human rights of individuals rather than the rights of states, which can both widen and narrow what is permitted, for example in humanitarian intervention or in the treatment of civilians who contribute to a war.

Nonviolence

Tolstoy read the Sermon on the Mount as an absolute command not to resist evil with violence, and Gandhi turned nonviolent resistance – satyagraha, “holding firmly to truth” – into a political method that King adopted in the American civil rights movement. Contemporary “contingent pacifists” such as Larry May (2015) do not hold that violence is wrong in principle, but argue that modern wars, with their inevitable killing of civilians, practically never meet the conditions of just war theory. They point to research by Erica Chenoweth and Maria Stephan (Why Civil Resistance Works, 2011), which found that between 1900 and 2006 nonviolent campaigns against regimes and occupations succeeded about twice as often as violent ones. Critics reply that against an invader who deports and kills, renouncing armed defence leaves the victims to their fate.

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Realism

Realists hold that relations between states are governed by power and security rather than morality. In Thucydides’ Melian Dialogue the Athenians tell the people of Melos that “the strong do what they can and the weak suffer what they must”, and Hobbes described rulers as standing towards one another in “a posture of war”, since no power above them can enforce agreements. Modern realists such as Hans Morgenthau do not deny morality but warn against crusades: a war fought in the name of justice tends to become total, whereas prudent statecraft that respects the balance of power and the security interests of rivals keeps the peace more reliably. Their critics object that such advice too easily rewards the aggressor and disregards the right of smaller nations to choose their own alliances.

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Peace through law

Kant’s Perpetual Peace (1795) argued that peace is not a natural condition but must be established: through republican constitutions, whose citizens would hesitate to take the burdens of war upon themselves, a federation of free states and a cosmopolitan right. Habermas developed this into the project of a “constitutionalisation” of international law. After Russia’s invasion he argued (2022, 2023) that the West must support Ukraine but also weigh the danger of escalation into a war with a nuclear power, and urged that negotiations be sought early – a stance many critics found too hesitant. Russell and Einstein had warned in their manifesto of 1955 that in the nuclear age humanity must renounce war or perish, and Galtung distinguished the “negative peace” of an absence of war from a “positive peace” that overcomes the structural violence of poverty and oppression.

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As of 10/2026. Summaries for orientation – the sources give the full arguments. · How Agora is made

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