An army advancing against the enemy, with the design of bringing him to a general action, if it has to pass over an unoccupied range of mountain, has naturally to apprehend that the enemy may, at the last moment, block up those very passes which it proposes to use on its march: in such a case, the assailant will by no means have the same advantages as if the enemy occupied merely an ordinary mountain position. The latter is, for instance, not then in a position extended beyond measure, nor is he in uncertainty as to the road which the assailant will take; the assailant has not been able to choose his road with reference to the enemy’s position, and therefore this battle in the mountains is not then united with all those advantages on his side of which we have spoken in the sixth book; under such circumstances, the defender might be found in an impregnable position—According to this, the defender might even have means at his command of making advantageous use of the mountains for a great battle.—This is, at any rate, possible; but if we reflect on the difficulties which the defender would have to encounter in establishing himself in a strong position in the mountains just at the last moment, particularly if he has left it entirely unoccupied before, we may put down this means of defence as one upon which no dependence can be placed, and therefore as one, the probability of which the assailant has little reason to dread. But even if it is a very improbable case, yet still it is natural to fear it; for in war, many a thing is very natural, and yet in a certain measure superfluous.
But another measure which the assailant has to apprehend here is, a preliminary defence of the mountains by an advanced guard or chain of outposts. This means, also, will seldom accord with the interests of the defender; but the assailant has not the means of discerning how far it may be beneficial to the defender or otherwise, and therefore he has only to provide against the worst.
Further, our view by no means excludes the possibility of a position being quite unassailable from the mountainous character of the ground: