Under these conditions, therefore, an army on the defensive, posted behind a large river or deep valley with steep sides, is in a very excellent position, and this sort of river defence is a strategic measure of the best kind.
Its defect (the point on which the defender is very apt to err) is the over-extension of the defending force. It is so natural in such a case to be drawn on from one point of passage to another, and to miss the right point where we ought to stop; but then, if we do not succeed in fighting with the whole army united, we miss the intended effect; a defeat in battle, the necessity of retreat, confusion in many ways and losses reduce the army nearly to ruin, even although the resistance has not been pushed to an extremity.
In saying that the defensive, under the above conditions, should not extend his forces widely, that he should be in any case able to assemble all his forces on the evening of the day on which the enemy passes, enough is said, and it may stand in place of all combinations of time, power, and space, things which, in this case, must depend on many local points.
The battle to which these circumstances lead must have a special character—that of the greatest impetuosity on the side of the defender. The feigned passages by which the enemy will keep him for some time in uncertainty—will, in general prevent his discovering the real point of crossing a moment too soon. The peculiar advantages of the situation of the defender consist in the disadvantageous situation of the enemy’s corps just immediately in his front; if other corps, having passed at other points, menace his flank, he cannot, as in a defensive battle, counteract such movements by vigorous blows from his rear, for that would be to sacrifice the above-mentioned advantage of his situation; he must, therefore, decide the affair in his front before such other corps can arrive and become dangerous, that is, he must attack what he has before him as swiftly and vigorously as possible, and decide all by its defeat.