The moral reality of war is divided into two parts. War is always judged twice, first with reference to the reasons states have for fighting, secondly with reference to the means they adopt.... The two sorts of judgment are logically independent. It is perfectly possible for a just war to be fought unjustly and for an unjust war to be fought in strict accordance with the rules. But this independence, though our views of particular wars often conform to its terms, is nevertheless puzzling. It is a crime to commit aggression, but aggressive war is a rule-governed activity. It is right to resist aggression, but the resistance is subject to moral (and legal) restraint. [This] dualism ... is at the heart of all that is most problematic in the moral reality of war
MICHAEL WALZER, Just and Unjust Wars