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.

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