For the past century or so, war has not been a major topic in political philosophy, but Prof. Walzer (Government, Harvard) sees it as something that must be dealt with in moral, rather than policy-making terms. Refuting the contention that war is an amoral activity, he demonstrates that moral issues always enter into the decision to go to war, even into the definition of necessity ordinarily used to remove war from the moral sphere. Citing the famous example of Athens' war upon Melos, which was dictated by ""a necessity of nature"" (Thucydides), he reconstructs the debate in the Athenian assembly and the Melian argument that the war was unjust, exposing the moral issues on both sides and defending the Melian claim. This determination leads Walzer to a conception of the just war, which he derives from a theory of aggression based upon national fights (pertaining to sovereignty and territorial integrity). Resistance to aggression is just, even if that resistance takes the form of a first strike--thus the Israelis were justified in the Six-Day War, while the German invasions of Czechoslovakia and Poland, or the Russian invasion of Finland, were unjust, There are cases of wars which are unjust on all sides but no war can be just on both sides. States have the right, according to Walzer, to intervene in defense of another state which has been the victim of aggression, in order to punish the aggressor. But these general precepts mask the difficult moral judgments which must enter into the analysis of each individual act of war: at what point, if any, does resistance or intervention itself become aggression? how much punishment is just? Walzer approaches each such question and the more difficult daily ones, with historical illustrations ranging from the killing of prisoners by Henry V at Agincourt to the My Lai massacre. His case-study approach accentuates the difficulty of developing an all-encompassing moral theory of war--which is partly his point: each act must be fully assessed for its moral impact, there are no easy answers. But Walzer does show us how to proceed, and brilliantly clarifies the issues involved. On a par with Rawls' A Theory of Justice.