And what guarantee was there that the next war would confirm to the Napoleonic type and accommodate itself to the abstract theory? "This theory," he says, "is still quite powerless against the force of circumstances." And so it proved, for the wars of the middle nineteenth century did in fact revert to the pre-Napoleonic type. In short, Clausewitz's difficulty in adopting his abstract theory as a working rule was that his practical mind could not forget that war had not begun with the Revolutionary era, nor was it likely to end with it. If that era had changed the conduct of war, it must be presumed that war would change again with other times and other conditions. A theory of war which did not allow for this and did not cover all that had gone before was no theory at all. If a theory of war was to be of any use as a practical guide it must cover and explain not only the extreme manifestation of hostility which he himself had witnessed, but every manifestation that had occurred in the past or was likely to recur in the future. 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.