I will argue that Buchanan’s argument against the principle that war is permissible only in response to an actual or imminent attack rests on a mistaken understanding of the nature and purpose of the JWN. Buchanan abstracts from the fact that the JWN is not just a moral principle but also a legal rule and, as such, part of an already existing institutionalized system for the regulation of the use of force. Due to this abstraction, he fails to take into account the JWN’s role as a fundamental constitutional principle in an already existing society of equal states committed to the values of peace and equality among states. It follows that Buchanan’s own argument is arbitrarily incomplete since it mistakenly reduces the JWN to a mere safeguard against miscalculation and manipulation in an imaginary state of nature.