The case of an army drawn up with its front close on a river, or stream, or deep valley, in order by that means to command a tactical obstacle to the approach to its position, or to strengthen its front, is quite a different one, the detailed examination of which belongs to tactics. Of the effect of this we shall only say this much, that it is founded on a delusion.—If the cleft in the ground is very considerable, the front of the position becomes absolutely unassailable. Now, as there is no more difficulty in passing round such a position than any other, it is just the same as if the defender had himself gone out of the way of the assailant, yet that could hardly be the object of the position. A position of this kind can, therefore, only be advisable when, as a consequence of its position, it threatens the communications of the assailant, so that every deviation by him from the direct road is fraught with consequences altogether too serious to be risked.
In this second form of defence, feigned passages are much more dangerous, for the assailant can make them more easily, while, on the other hand, the proposition for the defender is, to assemble his whole army at the right point. But the defender is certainly not quite so much limited for time here, because the advantage of his situation lasts until the assailant has massed his whole force, and made himself master of several crossings; moreover, also, the simulated attack has not the same degree of effect here as in the defence of a cordon, where all must be held, and where, therefore, in the application of the reserve, it is not merely a question, as in our proposition, where the enemy has his principal force, but the much more difficult one, Which is the point he will first seek to force?