Book 1, Chapter 1, Section 24 (page 87)
This, to me, is the most beautiful insight that Sir Carl left to us, and lays the bedrock for the proper civil-military relationship. In fact, I'd argue that Eliot Cohen's thesis in Supreme Command takes it cue precisely from this particular passage from On War, and instead of being controversial, it should be regarded as the correct view.We see, therefore, that war is not merely an act of policy but a true political instrument, a continuation of political intercourse, carried on with other means. What remains peculiar to war is simply the peculiar nature of its means. War in general, and the commander in any specific instance, is entitled to require that the trend and designs of policy shall not be inconsistent with these means. That, of course, is no small demand; but however much it may affect political aims in a given case, it will never do more than modify them. The political object is the goal, war is the means of reaching it, and means can never be considered in isolation from their purpose.