On War

by Carl von Clausewitz

In the fifteenth chapter we spoke of the nature of combats in mountains, and in the sixteenth of the use to be made of them by strategy, and in so doing we often came upon the idea of mountain defence, without stopping to consider the form and details of such a measure. We shall now examine it more closely.

As mountain systems frequently extend like streaks or belts over the surface of the earth, and form the division between streams flowing in different directions, consequently the separation between whole water systems, and as this general form repeats itself in the parts composing that whole, inasmuch as these parts diverge from the main chain in branches or ridges, and then form the separation between lesser water systems; hence the idea of a system of mountain defence has naturally founded itself in the first instance, and afterwards developed itself, upon the conception of the general form of mountains, that of an obstacle, like a great barrier, having greater length than breadth. Although geologists are not yet agreed as to the origin of mountains and the laws of their formation, still in every case the course of the waters indicates in the shortest and surest manner the general form of the system, whether the action of the water has contributed to give that general form (according to the aqueous theory), or that the course of the water is a consequence of the form of the system itself. It was, therefore, very natural again, in devising a system of mountain defence, to take the course of the waters as a guide, as those courses form a natural series of levels, from which we can obtain both the general height and the general profile of the mountain, while the valleys formed by the streams present also the best means of access to the heights, because so much of the effect of the erosive and alluvial action of the water is permanent, that the inequalities of the slopes of the mountain are smoothed down by it to one regular slope. Hence, therefore, the idea of mountain defence would assume that, when a mountain ran about parallel with the front to be defended, it was to be regarded as a great obstacle to approach, as a kind of rampart, the gates of which were formed by the valleys. The real defence was then to be made on the crest of this rampart, (that is, on the edge of the plateau which crowned the mountain) and cut the valleys transversely. If the line of the principal mountain-chain formed somewhat of a right angle with the front of defence, then one of the principal branches would be selected to be used instead; thus the line chosen would be parallel to one of the principal valleys, and run up to the principal ridge, which might be regarded as the extremity.

We have noticed this scheme for mountain defence founded on the geological structure of the earth, because it really presented itself in theory for some time, and in the so-called “theory of ground” the laws of the process of aqueous action have been mixed up with the conduct of war.

But all this is so full of false hypotheses and incorrect substitutions, that when these are abstracted, nothing in reality remains to serve as the basis of any kind of a system.

The principal ridges of real mountains are far too impracticable and inhospitable to place large masses of troops upon them; it is often the same with the adjacent ridges, they are often too short and irregular. Plateaux do not exist on all mountain ridges, and where they are to be found they are mostly narrow, and therefore unfit to accommodate many troops; indeed, there are few mountains which, closely examined, will be found surmounted by an uninterrupted ridge, or which have their sides at such an angle that they form in some measure practicable slopes, or, at least, a succession of terraces. The principal ridge winds, bends, and splits itself; immense branches launch into the adjacent country in curved lines, and lift themselves often just at their termination to a greater height than the main ridge itself; promontories then join on, and form deep valleys which do not correspond with the general system. Thus it is that, when several lines of mountains cross each other, or at those points from which they branch out, the conception of a small band or belt is completely at an end, and gives place to mountain and water lines radiating from a centre in the form of a star.

From this it follows, and it will strike those who have examined mountain-masses in this manner the more forcibly, that the idea of a systematic disposition is out of the question, and that to adhere to such an idea as a fundamental principle for our measures would be wholly impracticable. There is still one important point to notice belonging to the province of practical application.

If we look closely at mountain warfare in its tactical aspects, it is evident that these are of two principal kinds, the first of which is the defence of steep slopes, the second is that of narrow valleys. Now this last, which is often, indeed almost generally, highly favourable to the action of the defence, is not very compatible with the disposition on the principal ridge, for the occupation of the valley itself is often required and that at its outer extremity nearest to the open country, not at its commencement, because there its sides are steeper. Besides, this defence of valleys offers a means of defending mountainous districts, even when the ridge itself affords no position which can be occupied; the rôle which it performs is, therefore, generally greater in proportion as the masses of the mountains are higher and more inaccessible.

The result of all these considerations is, that we must entirely give up the idea of a defensible line more or less regular, and coincident with one of the geological lines, and must look upon a mountain range as merely a surface intersected and broken with inequalities and obstacles strewed over it in the most diversified manner, the features of which we must try to make the best use of which circumstances permit; that therefore, although a knowledge of the geological features of the ground is indispensable to a clear conception of the form of mountain masses, it is of little value in the organisation of defensive measures.

Neither in the war of the Austrian Succession, nor in the Seven Years’ War, nor in those of the French Revolution, do we find military dispositions which comprehended a whole mountain system, and in which the defence was systematised in accordance with the leading features of that system. Nowhere do we find armies on the principal ridges always in position on the slopes. Sometimes at a greater, sometimes at a lower elevation; sometimes in one direction, sometimes in another; parallel, at right angles, and obliquely; with and against the watercourse; in lofty mountains, such as the Alps, frequently extended along the valleys; amongst mountains of a inferior class, like the Sudetics (and this is the strangest anomaly), at the middle of the declivity, as it sloped towards the defender, therefore with the principal ridge in front, like the position in which Frederick the Great, in 1762, covered the siege of Schwednitz, with the “hohe Eule” before the front of his camp.

The celebrated positions, Schmotseifen and Landshut, in the Seven Years’ War, are for the most part in the bottoms of valleys. It is the same with the position of Feldkirch, in the Vorarlsberg. In the campaigns of 1799 and 1800, the chief posts, both of the French and Austrians, were always quite in the valleys, not merely across them so as to close them, but also parallel with them, whilst the ridges were either not occupied at all, or merely by a few single posts.