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In order for a war to be just, three things are necessary. First, the authority of the sovereign…. Secondly, a just cause…. Thirdly … a rightful intention.

— Thomas Aquinas

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Source: Summa Theologica (1266–1273; 1947 republication), part II–II, question 40, article 1, p. 1359–60. The three conditions are sometimes paraphrased as: public authority, just cause, right motive.
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War

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Quote text and notes from Wikiquote: Thomas Aquinas (page history), available under CC BY-SA 4.0. Source quality and labels are our own summary of what Wikiquote records.