On War

by Carl von Clausewitz

CHAPTER X.
Fortresses

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, corps 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 bodies of troops 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 corps 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 role 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.