The process by which he reached his famous theory can be followed in his classical work On War and the Notes regarding it which he left behind him.  In accordance with the philosophic fashion of his time he began by trying to formulate an abstract idea of war.  The definition he started with was that “War is an act of violence to compel our opponent to do our will.”  But that act of violence was not merely “the shock of armies,” as Montecuccoli had defined it a century and a half before.  If the abstract idea of war be followed to its logical conclusion, the act of violence must be performed with the whole of the means at our disposal and with the utmost exertion of our will.  Consequently we get the conception of two armed nations flinging themselves one upon the other, and continuing the struggle with the utmost strength and energy they can command till one or other is no longer capable of resistance.  This Clausewitz called “Absolute War.”  But his practical experience and ripe study of history told him at once that “Real War” was something radically different.  It was true, as he said, that Napoleon’s methods had approximated to the absolute and had given some colour to the use of the absolute idea as a working theory.  “But shall we,” he acutely asks, “rest satisfied with this idea and judge all wars by it however much they may differ from it—­shall we deduce from it all the requirements of theory?  We must decide the point, for we can say nothing trustworthy about a war plan until we have made up our minds whether war should only be of this kind or whether it may be of another kind.”  He saw at once that a theory formed upon the abstract or absolute idea of war would not cover the ground, and therefore failed to give what was required for practical purposes.  It would exclude almost the whole of war from Alexander’s time to Napoleon’s.  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.