On War

by Carl von Clausewitz

We shall now proceed to examine these subjects a little more closely.

At first sight it appears somewhat preposterous to protect a fortress from the enemy’s attack by placing an army in front of it; such a measure looks like a kind of pleonasm, as fortifications are built to resist a hostile attack of themselves. Yet it is a measure which we see resorted to thousands and thousands of times. But thus it is in the conduct of war; the most common things often seem the most incomprehensible. Who would presume to pronounce these thousands of instances to be so many blunders on the ground of this seeming inconsistency? The constant repetition of the measure shows that it must proceed from some deep-seated motive. This reason is, however, no other than that pointed out above, emanating from moral sluggishness and inactivity.

If the defender places himself in front of his fortress, the enemy cannot attack it unless he first beats the army in front of it; but a battle is a decision; if that is not the enemy’s object then there will be no battle, and the defender will remain in possession of his fortress without striking a blow; consequently, whenever we do not believe the enemy intends to fight a battle, we should venture on the chance of his not making up his mind to do so, especially as in most cases we still retain the power of withdrawing behind the fortress in a moment, if, contrary to our expectation, the enemy should march to attack us; the position before the fortress is in this way free from danger, and the probability of maintaining the status quo without any sacrifice, is not even attended with the slightest risk.

If the defender places himself behind the fortress, he offers the assailant an object which is exactly suited to the circumstances in which the latter is placed. If the fortress is not of great strength, and he is not quite unprepared, he will commence the siege: in order that this may not end in the fall of the place, the defender must march to its relief. The positive action, the initiative, is now laid on him, and the adversary who by his siege is to be regarded as advancing towards his object, is in the situation of occupier.

Experience teaches that the matter always takes this turn, and it does so naturally. A catastrophe, as we have before said, is not necessarily bound up with a siege. Even a general, devoid of either the spirit of enterprise or energy, who would never make up his mind to a battle, will proceed to undertake a siege with perhaps nothing but field artillery, when he can approach a fortress without risk. At the worst he can abandon his undertaking without any positive loss. There always remains to be considered the danger to which most fortresses are more or less exposed, that of being taken by assault, or in some other irregular manner, and this circumstance should certainly not be overlooked by the defender in his calculation of probabilities.

In weighing and considering the different chances, it seems natural that the defender should look upon the probability of not having to fight at all as more for his advantage than the probability of fighting even under favourable circumstances. And thus it appears to us that the practice of placing an army in the field before its fortress, is both natural and fully explained. Frederick the Great, for instance, at Glogau, against the Russians, at Schwednitz, Neiss, and Dresden, against the Austrians, almost always adopted it. This measure, however, brought misfortune on the Duke of Bevern at Breslau; behind Breslau he could not have been attacked; the superiority of the Austrians in the king’s absence would soon cease, as he was approaching; and therefore, by a position behind Breslau, a battle might have been avoided until Frederick’s arrival. No doubt the Duke would have preferred that course if it had not been that it would have exposed that important place to a bombardment, at which the king, who was anything but tolerant on such occasions, would have been highly displeased. The attempt made by the Duke to protect Breslau by an entrenched position taken up for the purpose, cannot after all be disapproved, for it was very possible that Prince Charles of Lorraine, contented with the capture of Schwednitz, and threatened by the march of the king, would, by that position, have been prevented from advancing farther. The best thing he could have done would have been to refuse the battle at the last by withdrawing through Breslau at the moment that the Austrians advanced to the attack; in this way he would have got all the advantages of the state of expectation without paying for them by a great danger.

If we have here traced the position before a fortress to reasons of a superior and absolute order, and defended its adoption on those grounds, we have still to observe that there is a motive of a secondary class which, though a more obvious one, is not sufficient of itself alone, not being absolute; we refer to the use which is made by armies of the nearest fortress as a depôt of provisions and munitions of war. This is so convenient, and presents so many advantages, that a general will not easily make up his mind to draw his supplies of all kinds from more distant places, or to lodge them in open towns. But if a fortress is the great magazine of an army, then the position before it is frequently a matter of absolute necessity, and in most cases is very natural. But it is easy to see that this obvious motive, which is easily over-valued by those who are not in the habit of looking far before them, is neither sufficient to explain all cases, nor are the circumstances connected with it of sufficient importance to entitle it to give a final decision.

The capture of one or more fortresses without risking a battle, is such a very natural object of all attacks which do not aim at a decision on the field of battle, that the defender makes it his principal business to thwart this design. Thus it is that on theatres of war, containing a number of fortresses, we find these places made the pivots of almost all the movements; we find the assailant seeking to approach one of them unexpectedly, and employing various feints to aid his purpose, and the defender immediately seeking to stop him by well-prepared movements. Such is the general character of almost all the campaigns of Louis XIV. in the Netherlands up to the time of Marshal Saxe.

So much for the covering of fortresses.

The covering of a country by an extended disposition of forces, is only conceivable in combination with very considerable obstacles of ground. The great and small posts which must be formed for the purpose, can only get a certain capability of resistance through strength of position; and as natural obstacles are seldom found sufficient, therefore field fortification is made use of as an assistance. But now it is to be observed that, the power of resistance which is thus obtained at any one point, is always only relative (see the chapter on the signification of the combat), and never to be regarded as absolute. It may certainly happen that one such post may remain proof against all attacks made upon it, and that therefore in a single instance there may be an absolute result; but from the great number of posts, any single one, in comparison to the whole, appears weak, and exposed to the possible attack of an overwhelming force, and consequently it would be unreasonable to place one’s dependence for safety on the resistance of any one single post. In such an extended position, we can therefore only count on a resistance of relative length, and not upon a victory, properly speaking. This value of single posts, at the same time, is also sufficient for the object, and for a general calculation. In campaigns in which no great decision, no irresistible march, towards the complete subjugation of the whole force is to be feared, there is little risk in a combat of posts, even if it ends in the loss of a post. There is seldom any further result in connection with it than the loss of the post and a few trophies; the influence of victory penetrates no further into the situation of affairs, it does not tear down any part of the foundation to be followed by a mass of building in ruin. In the worst case, if, for instance, the whole defensive system is disorganised by the loss of a single post, the defender has always time to concentrate his corps, and with his whole force to offer battle, which the assailant, according to our supposition, does not desire. Therefore also it usually happens that with this concentration of force the act closes, and the further advance of the assailant is stopped. A strip of land, a few men and guns, are the losses of the defender, and with these results the assailant is satisfied.