Knowledge must be Thorough
This Clausewitz considers most important. He says that "Knowledge of the conduct of war ... must pass completely into the mind, and almost cease to be something objective." For in war "The moral reaction, the ever-changing form of things makes it necessary for the chief actor to carry in himself the whole mental apparatus of his knowledge, in order that anywhere and at every pulse-beat he may be capable of giving the requisite decision from himself. Knowledge must, by this complete assimilation with his own mind and life, be converted into real power."
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So much for Clausewitz, therefore, as the greatest yet the simplest and least theoretical of theorists on war. Mark well his comforting dictum that "Theory is nothing but rational reflection upon all the situations in which we can be placed in war." That is a task which we have all more or less attempted. Therefore we are all more or less theorists. The only question is that of comparative "thoroughness" in our reflections. And it is essentially this "thoroughness" in investigation and reflection towards which Clausewitz helps us. Like every other habit, the habit of military reflection gradually grows with use; till, fortified and strengthened by detailed knowledge, it gradually becomes Power.