On War

by Carl von Clausewitz

We need hardly observe that we speak of the strategic flank, that is, a side of the theatre of war, and that the attack from one side in battle, or the tactical movement against a flank, must not be confounded with it; and even in cases in which the strategic operation against a flank, in its last stage, ends in the tactical operation, they can quite easily be kept separate, because the one never follows necessarily out of the other.

These flanking movements, and the flanking positions connected with them, belong also to the mere useless pageantry of theory, which is seldom met with in actual war. Not that the means itself is either ineffectual or illusory, but because both sides generally seek to guard themselves against its effects; and cases in which this is impossible are rare. Now in these uncommon cases this means has often also proved highly efficacious, and for this reason, as well as on account of the constant watching against it which is required in war, it is important that it should be clearly explained in theory. Although the strategic operation against a flank can naturally be imagined, not only on the part of the defensive, but also on that of the offensive, still it has much more affinity with the first, and therefore finds its place under the head of defensive means.

Before we enter into the subject, we must establish the simple principle, which must never be lost sight of afterwards in the consideration of the subject, that troops which are to act against the rear or flank of the enemy cannot be employed against his front, and that, therefore, whether it be in tactics or strategy, it is a completely false kind of notion to consider that coming on the rear of the enemy is at once an advantage in itself. In itself, it is as yet nothing; but it will become something in connection with other things, and something either advantageous or the reverse, according to the nature of these things, the examination of which now claims our attention.

First, in the action against the strategic flank, we must make a distinction between two objects of that measure—between the action merely against the communications, and that against the line of retreat, with which, at the same time, an effect upon the communications may also be combined.

When Daun, in 1758, sent a detachment to seize the convoys on their way to the siege of Olmütz, he had plainly no intention of impeding the king’s retreat into Silesia; he rather wished to bring about that retreat, and would willingly have opened the line to him.

In the campaign of 1812, the object of all the expeditionary corps that were detached from the Russian army in the months of September and October, was only to intercept the communications, not to stop the retreat; but the latter was quite plainly the design of the Moldavian army which, under Tschitschagof, marched against the Beresina, as well as of the attack which General Wittgenstein was commissioned to make on the French corps stationed on the Dwina.

These examples are merely to make the exposition clearer.

The action against the lines of communication is directed against the enemy’s convoys, against small detachments following in rear of the army, against couriers and travellers, small depôts, etc.; in fact, against all the means which the enemy requires to keep his army in a vigorous and healthy condition; its object is, therefore, to weaken the condition of the enemy in this respect, and by this means to cause him to retreat.

The action against the enemy’s line of retreat is to cut his army off from that line. It cannot effect this object unless the enemy really determines to retreat; but it may certainly cause him to do so by threatening his line of retreat, and, therefore, it may have the same effect as the action against the line of communication, by working as a demonstration. But as already said, none of these effects are to be expected from the mere turning which has been effected, from the mere geometrical form given to the disposition of the troops, they only result from the conditions suitable to the same.

In order to learn more distinctly these conditions, we shall separate completely the two actions against the flank, and first consider that which is directed against the communications.