Lesson · 4 min Stretch
When Is War Just?
In one line Just war theory asks when going to war can be right and how it must be fought; pacifists reject war altogether.
Just war theory, developed by Augustine, Aquinas and later thinkers, holds that war is sometimes permitted but always needs strict justification. It has two parts. Jus ad bellum asks when going to war is right: there must be a just cause such as defense, right intention, legitimate authority, a reasonable chance of success, proportionality, and war as a last resort.
Jus in bello asks how war must be fought: combatants must be distinguished from civilians, and force must be proportionate to the military aim. A just cause does not license any means.
Pacifists, from early Christians to Tolstoy and Gandhi, reply that killing in war cannot be justified, and that nonviolent resistance can work. Realists reply that morality hardly applies between states at all.
Go deeper
Michael Walzer’s Just and Unjust Wars, from 1977, revived the theory with modern examples. Jeff McMahan has argued that soldiers fighting for an unjust cause are not morally equal to those resisting them, challenging the traditional view. Many add a third part, jus post bellum, asking what a just peace requires. Gandhi’s alternative is explored in Satyagraha.
The fence
Two neighbors have argued for years over a fence. One proposes tearing it down overnight to settle things.
His friend asks just-war-style questions: has every other option been tried? Will the action likely solve anything? Will it harm people who had nothing to do with the dispute? The questions are not only for nations. They are a checklist for any use of force.
Try this today Pick a conflict in the news. Ask which just war condition each side claims, and which it seems to fail.
Cite or share this lesson
A plain reference you can paste into notes or an essay:
Quest2Reason. “When Is War Just?.” https://quest2reason.com/learn/lesson/when-is-war-just. Accessed 13 November 2026.
Sources
- Stanford Encyclopedia of Philosophy, “War” — Covers just war theory, its revisions and its critics.
- Wikipedia, “Just war theory” — Summarizes the criteria from Augustine to Walzer.
- Wikipedia, “Pacifism” — Covers religious and secular pacifism, including Tolstoy and Gandhi.
- Michael Walzer, Just and Unjust Wars (1977) — The modern restatement of just war theory, with historical cases. Paraphrased; nothing is quoted.