Front Page Titles (by Subject) CHAPTER X: FORTRESSES - On War, vol. 2
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CHAPTER X: FORTRESSES - Carl von Clausewitz, On War, vol. 2 
On War, trans. Col. J.J. Graham. New and Revised edition with Introduction and Notes by Col. F.N. Maude, in Three Volumes (London: Kegan Paul, Trench, Trubner & C., 1918). Vol. 2.
Part of: On War
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Formerly, and up to the time of great standing Armies, fortresses, that is castles and fortified towns, were only built for the defence and protection of the inhabitants. The baron, if he saw himself pressed on all sides, took refuge in his castle to gain time and wait a more favourable moment; and towns sought by their walls to keep off the passing hurricane of War. This simplest and most natural object of fortresses did not continue to be the only one; the relation which such a place acquired with regard to the whole country and to troops acting here and there in the country soon gave these fortified points a wider importance, a signification which made itself felt beyond their walls, and contributed essentially to the conquest or occupation of the country, to the successful or unsuccessful issue of the whole contest, and in this manner they even became a means of making War more of a connected whole. Thus fortresses acquired that strategic significance which for a time was regarded as so important that it dictated the leading features of the plans of campaigns, which were more directed to the taking of one or more fortresses than the destruction of the enemy’s Army in the field. Men reverted to the cause of the importance of these places, that is to the connection between a fortified point, and the country, and the Armies; and then thought that they could not be sufficiently particular or too philosophical in choosing the points to be fortified. In these abstract objects the original one was almost lost sight of, and at length they came to the idea of fortresses without either towns or inhabitants.
On the other hand, the times are past in which the mere enclosure of a place with walls, without any military preparations, could keep a place dry during an inundation of War sweeping over the whole country. Such a possibility rested partly on the division of Nations formerly into small States, partly on the periodical character of the incursions then in vogue, which had fixed and very limited duration, almost in accordance with the seasons, as either the feudal forces hastened home, or the pay for the condottieri used regularly to run short. Since large standing Armies, with powerful trains of artillery mow down the opposition of walls or ramparts as it were with a machine, neither town nor other small corporation has any longer an inclination to hazard all their means only to be taken a few weeks or months later, and then to be treated so much the worse. Still less can it be the interest of an Army to break itself up into garrisons for a number of strong places, which may for a time retard the progress of the enemy, but must in the end submit. We must always keep enough forces, over and above those in garrison, to make us equal to the enemy in the open field, unless we can depend on the arrival of an Ally, who will relieve our strong places and set our Army free. Consequently the number of fortresses has necessarily much diminished, and this has again led to the abandonment of the idea of directly protecting the population and property in towns by fortifications, and promoted the other idea of regarding the fortresses as an indirect protection to the country, which they secure by their strategic importance as knots which hold together the strategic web.
Such has been the course of ideas, not only in books but also in actual experience. At the same time, as usually happens, it has been much more spun out in books.
Natural as was this tendency of things, still these ideas were carried out to an extreme, and mere crotchets and fancies displaced the sound core of a natural and urgent want. We shall look into these simple and important wants when we enumerate the objects and conditions of fortresses all together; we shall thereby advance from the simple to the more complicated, and in the succeeding chapter we shall see what is to be deduced therefrom as to the determination of the position and number of fortresses.
The efficacy of a fortress is plainly composed of two different elements, the passive and the active. By the first it shelters the place, and all that it contains; by the other it possesses a certain influence over the adjacent country, even beyond the range of its guns.
This active element consists in the attacks which the garrison may undertake upon every enemy who approaches within a certain distance. The larger the garrison, so much the stronger numerically will be the detachments that may be employed on such expeditions, and the stronger such detachments the wider as a rule will be the range of their operations; from which it follows that the sphere of the active influence of a great fortress is not only greater in intensity but also more extensive than that of a small one. But the active element itself is again, to a certain extent, of two kinds, consisting namely of enterprises of the garrison proper, and of enterprises which other bodies of troops, great and small, not belonging to the garrison but in co-operation with it, may be able to carry out. For instance, bodies which independently would be too weak to face the enemy, may through the shelter which, in case of necessity, the walls of a fortress afford them, be able to maintain themselves in the country, and to a certain extent to command it.
The enterprises which the garrison of a fortress can venture to undertake are always somewhat restricted. Even in the case of large places and strong garrisons, the detachments which can be employed on such operations are mostly inconsiderable as compared with the forces in the field, and their average sphere of action seldom exceeds a couple of days’ marches. If the fortress is small, the detachments it can send out are quite insignificant and the range of their activity will generally be confined to the nearest villages. But bodies which do not belong to the garrison, and therefore are not under the necessity of returning to the place, are thereby much more at liberty in their movements, and by their means, if other circumstances are favourable, the external zone of action of a fortress may be immensely extended. Therefore if we speak of the active influence of fortresses in general terms, we must always keep this feature of the same principally in view.
But even the smallest active element of the weakest garrison, is still essential for the different objects which fortresses are destined to fulfil, for strictly speaking even the most passive of all the functions of a fortress (defence against attack) cannot be imagined exclusive of that active agency. At the same time it is evident that amongst the different purposes which a fortress may have to answer generally, or in this or that moment, the passive element will be most required at one time, the active at another. The rôle which a fortress is to fulfil may be perfectly simple, and the action of the place will in such case be to a certain extent direct; it may be partly complicated, and the action then becomes more or less indirect. We shall examine these subjects separately, commencing with the first; but at the outset we must state that a fortress may be intended to answer several of these purposes, perhaps all of them, either at once, or at least at different stages of the War.
We say, therefore, that fortresses are great and most important supports of the defensive.
1. As secure depôts of stores of all kinds. The assailant during his aggression subsists his Army from day to day; the defensive usually must have made preparations long beforehand, he need not therefore draw provisions exclusively from the district he occupies, and which he no doubt desires to spare. Storehouses are therefore for him a great necessity. The provisions of all kinds which the aggressor possesses are in his rear as he advances, and are therefore exempt from the dangers of the theatre of War, while those of the defensive are exposed to them. If these provisions of all kinds are not in fortified places, then a most injurious effect on the operations in the field is the consequence, and the most extended and compulsory positions often become necessary in order to cover depôts or sources of supply.
An Army on the defensive without fortresses has a hundred vulnerable spots; it is a body without armour.
2. As a protection to great and wealthy towns. This purpose is closely allied to the first, for great and wealthy towns, especially commercial ones, are the natural storehouses of an Army; as such their possession and loss affects the Army directly. Besides this, it is also always worth while to preserve this portion of the national wealth, partly on account of the resources which they furnish directly, partly because, in negotiations for peace, an important place is in itself a valuable weight thrown into the scale.
This use of fortresses has been too little regarded in modern times, and yet it is one of the most natural, and one which has a most powerful effect, and is the least liable to mistakes. If there was a country in which not only all great and rich cities, but all populous places as well were fortified, and defended by the inhabitants and the people belonging to the adjacent districts, then by that means the expedition of military operation would be so much reduced, and the people attacked would press with so great a part of their whole weight in the scales, that the talent as well as the force of will of the enemy’s General would sink to nothing.
We just mention this ideal application of fortification to a country to do justice to what we have just supposed to be the proper use of fortresses, and that the importance of the direct protection which they afford may not be overlooked for a moment; but in any other respect this idea will not again interrupt our considerations, for amongst the whole number of fortresses there must always be some which must be more strongly fortified than others, to serve as the real supports of the active Army.
The purposes specified under 1 and 2 hardly call forth any other but the passive action of fortresses.
3. As real barriers, they close the roads, and in most cases the rivers, on which they are situated.
It is not as easy as is generally supposed to find a practicable lateral road which passes round a fortress, for this turning must be made, not only out of reach of the guns of this place, but also by a detour greater or less, to avoid sorties of the garrison.
If the country is in the least degree difficult, there are often delays connected with the slightest deviation of the road which may cause the loss of a whole day’s march, and, if the road is much used, may become of great importance.
How they may have an influence on enterprises by closing the navigation of a river is clear in itself.
4. As tactical points d’appui. As the diameter of the zone covered by the fire of even a very inferior class of fortifications is usually some miles, fortresses may be considered always as the best points d’appui for the flanks of a position. A lake of several miles long is certainly an excellent support for the wing of an Army, and yet a fortress of moderate size is better. The flank does not require to rest close upon it, as the assailant, for the sake of his retreat, would not throw himself between our flank and that obstacle.
5. As a station (or stage). If fortresses are on the line of communication of the defensive, as is generally the case, they serve as halting places for all that passes up and down these lines. The chief danger to lines of communication is from irregular bands, whose action is always of the nature of a shock. If a valuable convoy, on the approach of such a comet, can reach a fortress by hastening the march or quickly turning, it is saved, and may wait there till the danger is past. Further, all troops marching to or from the Army, after halting here for a few days, are better able to hasten the remainder of the march, and a halting day is just the time of greatest danger. In this way a fortress situated half way on a line of communication of one hundred and fifty miles shortens the line in a manner one half.
6. As places of refuge for weak or defeated Corps. Under the guns of a moderate sized fortress every Corps is safe from the enemy’s blows, even if no entrenched camp is specially prepared for them. No doubt such a Corps must give up its further retreat if it waits too long; but this is no great sacrifice in cases where a further retreat would only end in complete destruction.
In many cases a fortress can ensure a few days’ halt without the retreat being altogether stopped. For the slightly wounded and fugitives who precede a beaten Army, it is especially suited as a place of refuge, where they can wait to rejoin their corps.
If Magdeburg had lain on the direct line of the Prussian retreat in 1806, and if that line had not been already lost at Auerstadt, the Army could easily have halted for three or four days near that great fortress, and rallied and reorganised itself. But even as it was it served as a rallying point for the remains of Hohenlohe’s Corps, which there first resumed the appearance of an Army.
It is only by actual experience in War itself that the beneficial influence of fortresses close at hand in disastrous times can be rightly understood. They contain powder and arms, forage and bread, give covering to the sick, security to the sound, and recovery of sense to the panic-stricken. They are like an hostelry in the desert.
In the four last named purposes it is evident that the active agency of fortresses is called more into requisition.
7. As a real shield against the enemy’s aggression. Fortresses which the defender leaves in his front break the stream of the enemy’s attack like ice breakers on the piers of a bridge. The enemy must at least invest them, and requires for that, if the garrisons are brave and enterprising, perhaps double their strength. But, besides, these garrisons may and do mostly consist in part of troops, who, although competent to duty in a garrison, are not fit for the field—half trained militia, invalids, convalescents, armed citizens, landsturm, &c. The enemy, therefore, in such case is perhaps weakened four times more than we are.
This disproportionate weakening of the enemy’s power is the first and most important but not the only advantage which a besieged fortress affords by its resistance. From the moment that the enemy crosses our line of fortresses, all his movements become much more constrained; he is limited in his lines of retreat, and must constantly attend to the direct covering of the sieges which he undertakes.
Here, therefore, fortresses co-operate with the defensive, act in a most extensive and decisive manner, and of all the objects that they can have, this may be regarded as the most important.
If this use of fortresses—far from being seen regularly repeating itself—occurs comparatively seldom in military history, the cause is to be found in the character of most Wars, this means being to a certain extent far too decisive and too thoroughly effectual for them, the explanation of which we leave till hereafter.
In this use of fortresses it is chiefly their offensive power that is called for, at least it is that by which their effectual action is chiefly produced. If a fortress was no more to an aggressor than a point which could not be occupied by him, it might be an obstacle to him, but not to such a degree as to compel him to lay siege to it. But as he cannot leave six, eight, or ten thousand men to do as they like in his rear, he is obliged to invest the place with a sufficient force, and if he desires that this investment should not continue to employ so large a detachment, he must convert the investment into a siege, and take the place. From the moment the siege commences, it is then chiefly the passive efficacy of the fortress which comes into action.
All the destinations of fortresses which we have been hitherto considering are fulfilled in a simple and mainly in a direct manner. On the other hand, in the next two objects the method of action is more complicated.
8. As a protection to extended cantonments. That a moderate-sized fortress closes the approach to cantonments lying behind it for a width of fifteen to twenty miles is a simple result of its existence; but how such a place comes to have the honour of covering a line of cantonments seventy-five to one hundred miles in length, which we find frequently spoken of in military history as a fact—that requires investigation as far as it has really taken place, and refutation so far as it may be mere illusion.
The following points offer themselves for consideration:—
(1.) That the place in itself blocks one of the main roads, and really covers a breadth of fifteen to twenty miles of country.
(2.) That it may be regarded as an exceptionally strong advanced post, or that it affords a more complete observation of the country, to which may be added facilities in the way of secret information through the ordinary relations of civil life which exist between a great town and the adjacent districts. It is natural that in a place of six, eight, or ten thousand inhabitants, one should be able to learn more of what is going on in the neighbourhood than in a mere village, the quarters of an ordinary outpost.
(3.) That smaller bodies are pivoted on it, derive from it protection and security, and from time to time can advance towards the enemy, it may be to bring in intelligence, or, in case he attempts to turn the fortress, to undertake something against his rear; that therefore although a fortress cannot quit its place, still it may have the efficacy of an advanced corps (Fifth Book, eighth Chapter).
(4.) That the defender, after assembling his troops, can take up his position at a point directly behind this fortress, which the assailant cannot reach without becoming exposed to danger from leaving the fortress in his rear.
No doubt every attack on a line of cantonments as such is to be taken in the sense of a surprise, or rather, we are only speaking here of that kind of attack; but it is evident in itself that an attack by surprise accomplishes its effect in a much shorter space of time than a regular attack on a theatre of War. Therefore, although in the latter case, a fortress which is to be passed by must necessarily be invested and kept in check, this investment will not be so indispensable in the case of a mere sudden attack on cantonments, and therefore in the same proportion the fortress will be less an obstacle to the attack of the cantonments. That is true enough; also the cantonments lying at a distance of thirty to forty miles from the fortress cannot be directly protected by it; but the object of such a sudden attack does not consist alone in the attack of a few cantonments. Until we reach the book on attack we cannot describe circumstantially the real object of such a sudden attack and what may be expected from it; but this much we may say at present, that its principal results are obtained, not by the actual attack on some isolated quarters, but by the series of combats which the aggressor forces on isolated detachments not in proper order, and more bent upon hurrying to certain points than upon fighting. But this attack and pursuit will always be in a direction more or less towards the centre of the enemy’s cantonments, and, therefore, an important fortress lying in front of this centre will certainly prove a very great impediment to the attack.
If we reflect on these four points in the whole of their effects, we see that an important fortress in a direct and in an indirect way certainly gives some security to a much greater extent of cantonments than we should think at first sight. “Some security” we say, for all these indirect agencies do not render the advance of the enemy impossible; they only make it more difficult, and a more serious consideration; consequently less probable and less of a danger for the defensive. But that is also all that was required, and all that should be understood in this case under the term “covering.” The real direct security must be attained by means of outposts and the arrangement of the cantonments themselves.
There is, therefore, some truth in ascribing to a great fortress the capability of covering a wide extent of cantonments lying in rear of it; but it is also not to be denied that often in plans of real campaigns, but still oftener in historical works, we meet with vague and empty expressions, or illusory views in connection with this subject. For if that covering is only realised by the co-operation of several circumstances, if it then also only produces a diminution of the danger, we can easily see that, in particular cases, through special circumstances, above all, through the boldness of the enemy, this whole covering may prove an illusion, and therefore in actual war we must not content ourselves with assuming hastily at once the efficacy of such and such a fortress, but carefully examine and study each single case on its own merits.
9. As covering a province not occupied. If during War a province is either not occupied at all, or only occupied by an insufficient force, and likewise exposed more or less to incursions from flying columns, then a fortress, if not too unimportant in size, may be looked upon as a covering, or, if we prefer, as a security for this province. As a security it may at all events be regarded, for an enemy cannot become master of the province until he has taken it, and that gives us time to hasten to its defence. But the actual covering can certainly only be supposed very indirect, or as not properly belonging to it. That is, the fortress by its active opposition can only in some measure check the incursions of hostile bands. If this opposition is limited to merely what the garrison can effect, then the result must be little indeed, for the garrisons of such places are generally weak and usually consist of infantry only, and that not of the best quality. The idea gains a little more reality if small columns keep themselves in communication with the place, making it their base and place of retreat in case of necessity.
10. As the focus of a general arming of the nation. Provisions, arms, and munitions can never be supplied in a regular manner in a People’s War; on the other hand, it is just in the very nature of such a War to do the best we can; in that way a thousand small sources furnishing means of resistance are opened which otherwise might have remained unused; and it is easy to see that a strong commodious fortress, as a great magazine of these things, can well give to the whole defence more force and intensity, more cohesion, and greater results.
Besides, a fortress is a place of refuge for wounded, the seat of the civil functionaries, the treasury, the point of assembly for the greater enterprises, &c., &c.; lastly, a nucleus of resistance which during the siege places the enemy’s force in a condition which facilitates and favours the attacks of national levies acting in conjunction.
11. For the defence of rivers and mountains. Nowhere can a fortress answer so many purposes, undertake to play so many parts, as when it is situated on a great river. It secures the passage at any time at that spot, and hinders that of the enemy for several miles each way, it commands the use of the river for commercial purposes, receives all ships within its walls, blocks bridges and roads, and helps the indirect defence of the river, that is, the defence by a position on the enemy’s side. It is evident that, by its influence in so many ways, it very greatly facilitates the defence of the river, and may be regarded as an essential part of that defence.
Fortresses in mountains are important in a similar manner. They there form the knots of whole systems of roads, which have their commencement and termination at that spot; they thus command the whole country which is traversed by these roads, and they may be regarded as the true buttresses of the whole defensive system.