On War

by Carl von Clausewitz

On the other hand, the third means, namely to attack the enemy in flank, therefore to give battle with a change of front, is attended with great advantages.

Firstly, there is always in this case, as we know, an exposure of the lines of communication, here the lines of retreat, and in this respect the defender has one advantage in his general relations as defender, and next and chiefly, the advantage which we have claimed for the strategic properties of his position at present.

Secondly,—and this is the principal thing,—every assailant who attempts to pass by his opponent is placed between two opposite tendencies. His first desire is to advance to attain the object of his attack; but the possibility of being attacked in flank at any moment, creates a necessity for being prepared, at any moment, to deliver a blow in that direction, and that too a blow with the mass of his forces. These two tendencies are contradictory, and beget such a complication in the internal relations (of his army), such a difficulty in the choice of measures, if they are to suit every event, that there can hardly be a more disagreeable position strategically. If the assailant knew with certainty the moment when he would be attacked, he might prepare to receive the enemy with skill and ability; but in his uncertainty on this point, and pressed by the necessity of advancing, it is almost certain that when the moment for battle arrives, it finds him in the midst of hurried and half-finished preparations, and therefore by no means in an advantageous relation to his enemy.

If then there are favourable moments for the defender to deliver an offensive battle, it is surely at such a moment as this, above all others, that we may look for success. If we consider, further, that the knowledge of the country and choice of ground are on the side of the defender, that he can prepare his movements, and can time them, no one can doubt that he possesses in such a situation a decided superiority, strategically, over his adversary.

We think, therefore, that a defender occupying a well chosen position, with his forces united, may quietly wait for the enemy passing by his army; should the enemy not attack him in his position, and that an operation against the enemy’s communications does not suit the circumstances, there still remains for him an excellent means of bringing about a decision by resorting to a flank attack.

If cases of this kind are hardly to be found in military history, the reason is, partly, that the defender has seldom had the courage to remain firm in such a position, but has either divided his forces, or rashly thrown himself in front of his enemy by a cross or diagonal march, or that no assailant dares to venture past the defender under such circumstances, and in that way his movement usually comes to a stand still.

The defender is in this case compelled to resort to an offensive battle: the further advantages of the state of expectation of a strong position, of good entrenchments, etc., etc., he must give up; in most cases the situation in which he finds the advancing enemy will not quite make up for these advantages, for it is just to evade their influence that the assailant has placed himself in his present situation; still it always offers him a certain compensation, and theory is therefore not just obliged to see a quantity disappear at once from the calculation, to see the pro and contra mutually cancel each other, as so often happens when critical writers of history introduce a little bit of theory.

It must not, in fact, be supposed that we are now dealing with logical subtilties; the subject is rather one which the more it is practically considered, the more it appears as an idea embracing the whole essence of defensive war, everywhere dominating and regulating it.

It is only by the determination on the part of the defender to assail his opponent with all his force, the moment he passes by him, that he avoids two pitfalls, close to which he is led by the defensive form; that is a division of his force, and a hasty flank march to intercept the assailant in front. In both he accepts the law of the assailant; in both he seeks to aid himself through measures of a very critical nature, and with a most dangerous degree of haste; and wherever a resolute adversary, thirsting for victory and a decision, has encountered such a system of defence, he has knocked it on the head. But when the defender has assembled his forces at the right point to fight a general action, if he is determined with this force, come what will, to attack his enemy in flank, he has done right, and is in the right course, and he is supported by all the advantages which the defence can give in his situation; his actions will then bear the stamp of good preparation, coolness, security, unity and simplicity.

We cannot here avoid mentioning a remarkable event in history, which has a close analogy with the ideas now developed; we do so to anticipate its being used in a wrong application.