The first man to enunciate a theory which would explain the phenomena of the Napoleonic era and co-ordinate them with previous history was General Carl von Clausewitz, a man whose arduous service on the Staff and the actual work of higher instruction had taught the necessity of systematising the study of his profession. He was no mere professor, but a soldier bred in the severest school of war. The pupil and friend of Sharnhorst and Gneisenau, he had served on the Staff of Bluecher in 1813, he had been Chief of the Staff to Wallmoden in his campaign against Davoust on the Lower Elbe, and also to the Third Prussian Army Corps in the campaign of 1815. Thereafter for more than ten years he was Director of the General Academy of War at Berlin, and died in 1831 as Chief of the Staff to Marshal Gneisenau. For the fifty years that followed his death his theories and system were, as he expected they would be, attacked from all sides. Yet to-day his work is more firmly established than ever as the necessary basis of all strategical thought, and above all in the "blood and iron" school of Germany. 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.