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Contemporary debate on humanitarian intervention has prompted a revival of interest in the tradition of moral reflection known as 'just war' (justum bellum). This tradition can be seen to provide an ethical vocabulary for assessing and possibly justifying these interventions. Just war is typically viewed as a middle way between pacifism, on the one hand, and realism, on the other; hence there is an ample body of literature comparing these traditions. Considerably less has been written, however, contrasting just war with perpetual peace. This article seeks to remedy that lacuna, with particular application to the question of humanitarian intervention. Taking the political controversy over NATO's 1999 Kosovo intervention as its point of departure, the article shows how support or opposition to this intervention often depended upon the differing presuppositions of the just war and perpetual peace traditions. The article then maps out these different presuppositions, with historical reference to exponents of each tradition, Aquinas and Vitoria for just war, Dante and Kant for perpetual peace.