On War

by Carl von Clausewitz

Here we must first establish two principal conditions, one or other of which must always be forthcoming.

The first is, that the forces used for this action against the flank of the enemy must be so insignificant in numbers that their absence is not observed in front.

The second, that the enemy’s army has run its career, and therefore can neither make use of a fresh victory over our army, nor can he pursue us if we evade a combat by moving out of the way.

This last case, which is by no means so uncommon as might be supposed, we shall lay aside for the moment, and occupy ourselves with the accessory conditions of the first.

The first of these is, that the communications have a certain length, and cannot be protected by a few good posts; the second point is, that the situation of the line is such as exposes it to our action.

This weakness of the line may arise in two ways—either by its direction, if it is not perpendicular to the strategic front of the enemy’s army, or because his lines of communication pass through our territory; if both these circumstances exist, the line is so much the more exposed. These two relations require a closer examination.

One would think that when it is a question of covering a line of communication forty or fifty miles long, it is of little consequence whether the position occupied by an army standing at one extremity of this line forms an oblique angle or a right angle in reference to it, as the breadth of the position is little more than a mere point in comparison to the line; and yet it is not so unimportant as it may seem. When an army is posted at a right angle with its communications, it is difficult, even with a considerable superiority, to interrupt the communications by any detachments or partisans sent out for the purpose. If we think only of the difficulty of covering absolutely a certain space, we should not believe this, but rather suppose, on the contrary, that it must be very difficult for an army to protect its rear (that is, the country behind it) against all expeditions which an enemy superior in numbers may undertake. Certainly, if we could look at everything in war as it is on a sheet of paper! Then the party covering the line, in his uncertainty as to the point where light troops or partisans may appear, would be in a certain measure blind, and only the partisans would see. But if we think of the uncertainty and insufficiency of intelligence gained in war, and know that both parties are incessantly groping in the dark, then we easily perceive that a detached corps sent round the enemy’s flank to gain his rear is in the position of a man engaged in a fray with numbers in a dark room. In the end he must fall; and so must it also be with bands who get round an army occupying a perpendicular position, and who therefore place themselves near to the enemy, but widely separated from their own people. Not only is there danger of losing numbers in this way; there is also a risk of the whole instrument itself being blunted immediately; for the very first misfortune which happens to one such party will make all the others timid, and instead of bold attacks and insolent dodging, the only play will be constant running away.

Through this difficulty, therefore, an army occupying a perpendicular position covers the nearest points on its line of communications for a distance of two or three marches, according to the strength of the army; but those nearest points are just those which are most in danger, as they are the nearest to the enemy.

On the other hand, in the case of a decidedly oblique position, no such part of the line of communication is covered; the smallest pressure, the most insignificant attempt on the part of the enemy, leads at once to a vulnerable point.

But now, what is it which determines the front of a position, if it is not just the direction perpendicular to the line of communication? The front of the enemy; but then, again, this may be equally as well supposed as dependent on our front. Here there is a reciprocal effect, for the origin of which we must search.