Cicero’s ethical and political writings present a detailed and sophisticated philosophy of just war, namely an account of when armed conflict is morally right or wrong. Several of the philosophical moves or arguments that he makes, such as a critique of “Roman realism” or his incorporation of the ius fetiale—a form of archaic international law—are remarkable similar to those of the contemporary just war philosopher Michael Walzer, even if Walzer is describing inter-state war and Cicero is describing imperial war. But if it is clear that Walzer presents a detailed philosophy of just war, then I argue we should draw the same conclusion for Cicero. The result is a deeper appreciation of the insight and novelty of Cicero’s view of just war. The paper concludes by arguing against the claim that Cicero’s philosophy of just war is derivative from the Stoic philosopher Panaetius, whom Cicero drew upon in the organization of his On Duties. Just as Walzer’s Just and Unjust Wars was written in response to America’s war in Vietnam, Cicero’s just war philosophy was written in response to the wars (both civil and external) of Gaius Caesar.