It was in casting about for the underlying causes of the oscillations manifested in the energy and intensity of hostile relations that he found his solution.  His experience on the Staff, and his study of the inner springs of war, told him it was never in fact a question of purely military endeavour aiming always at the extreme of what was possible or expedient from a purely military point of view.  The energy exhibited would always be modified by political considerations and by the depth of the national interest in the object of the war.  He saw that real war was in fact an international relation which differed from other international relations only in the method we adopted to achieve the object of our policy.  So it was he arrived at his famous theory—­“that war is a mere continuation of policy by other means.”

At first sight there seems little enough in it.  It may seem perhaps that we have been watching a mountain in labour and nothing but a mouse has been produced.  But it is only upon some such simple, even obvious, formula that any scientific system can be constructed with safety.  We have only to develop the meaning of this one to see how important and practical are the guiding lines which flow from it.

With the conception of war as a continuation of political intercourse before us, it is clear that everything which lies outside the political conception, everything, that is, which is strictly peculiar to military and naval operations, relates merely to the means which we use to achieve our policy.  Consequently, the first desideratum of a war plan is that the means adopted must conflict as little as possible with the political conditions from which the war springs.  In practice, of course, as in all human relations, there will be a compromise between the means and the end, between the political and the military exigencies.  But Clausewitz held that policy must always be the master.  The officer charged with the conduct of the war may of course demand that the tendencies and views of policy shall not be incompatible with the military means which are placed at his disposal; but however strongly this demand may react on policy in particular cases, military action must still be regarded only as a manifestation of policy.  It must never supersede policy.  The policy is always the object; war is only the means by which we obtain the object, and the means must always keep the end in view.

The practical importance of this conception will now become clear.  It will be seen to afford the logical or theoretical exposition of what we began by stating in its purely concrete form.  When a Chief of Staff is asked for a war plan he must not say we will make war in such and such a way because it was Napoleon’s or Moltke’s way.  He will ask what is the political object of the war, what are the political conditions, and how much does the question at issue mean respectively to us and to our adversary.  It is these considerations which determine the nature