The argument in this paper is that although rationality and morality are distinguishable concepts, there is nevertheless a rational morality, a set of principles, namely, which it is rational of all to require of all. The argument of this paper is that such a morality would certainly issue in a general condemnation of aggressive war. (Whether this also makes it irrational for States to engage in such activities is another, and not entirely settled, matter). Correlatively, it would issue in a strong right of defense. Would this right be sufficient to include resort to nuclear deterrence, if need be? It is argued that the answer must be in the affirmative - although the question of ‘need’ is by no means settled in current circumstances.