Front Page Titles (by Subject) CHAPTER XVII: DEFENCE OF MOUNTAINS ( Continued ) - On War, vol. 2
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CHAPTER XVII: DEFENCE OF MOUNTAINS ( Continued ) - 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.
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DEFENCE OF MOUNTAINS (Continued)
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 an 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.
The crests of the higher Alps in particular are so difficult of access, and afford so little space for the accommodation of troops, that it would be impossible to place any considerable bodies of men there. Now if we must positively have Armies in mountains to keep possession of them, there is nothing to be done but to place them in the valleys. At first sight this appears erroneous, because, in accordance with the prevalent theoretical ideas, it will be said, the heights command the valleys. But that is really not the case. Mountain ridges are only accessible by a few paths and rude tracks, with a few exceptions only passable for infantry, whilst the carriage roads are in the valleys. The enemy can only appear there at certain points with infantry; but in these mountain masses the distances are too great for any effective fire of small arms, and therefore a position in the valleys is less dangerous than it appears. At the same time, the valley defence is exposed to another great danger, that of being cut off. The enemy can, it is true, only descend into the valley with infantry, at certain points, slowly and with great exertion; he cannot, therefore, take us by surprise; but none of the positions we have in the valley defend the outlets of such paths into the valley. The enemy can, therefore, bring down large masses gradually, then spread out, and burst through the thin, and from that moment, weak line, which, perhaps, has nothing more for its protection than the rocky bed of a shallow mountain-stream. But now retreat, which must always be made piecemeal in a valley, until the outlet from the mountains is reached, is impossible for many parts of the line of troops; and that was the reason that the Austrians in Switzerland almost always lost a third, or a half of their troops taken prisoners.—
Now a few words on the usual way of dividing troops in such a method of defence.
Each of the subordinate positions is in relation with a position taken up by the principal body of troops, more or less in the centre of the whole line, on the principal road of approach. From this central position, other bodies are detached right and left to occupy the most important points of approach, and thus the whole is disposed in a line, as it were, of three, four, five, six posts, &c. How far this fractioning and extension of the line shall be carried, must depend on the requirements of each individual case. An extent of a couple of marches, that is, fifty to sixty miles is of moderate length, and we have seen it carried as far as one hundred or one hundred and fifty miles.
Between each of these separate posts, which are one or two leagues from each other, there will probably be some approaches of inferior importance, to which afterwards attention must be directed. Some very good posts for a couple of battalions each are selected, which form a good connection between the chief posts, and they are occupied. It is easy to see that the distribution of the force may be carried still further, and go down to posts occupied only by single companies and squadrons; and this has often happened. There are, therefore, in this no general limits to the extent of fractioning. On the other hand, the strength of each post must depend on the strength of the whole; and therefore we can say nothing as to the possible or natural degree which should be observed with regard to the strength of the principal posts. We shall only append, as a guide, some maxims which are drawn from experience and the nature of the case.
1. The more lofty and inaccessible the mountains are, so much the further this separation of divisions of the force not only may be, but also must be, carried; for the less any portion of a country can be kept secure by combinations dependent on the movement of troops, so much the more must the security be obtained by direct covering. The defence of the Alps requires a much greater division of force, and therefore approaches nearer to the cordon system, than the defence of the Vosges or the Giant mountains.
2. Hitherto, wherever defence of mountains has taken place, such a division of the force employed has been made that the chief posts have generally consisted of only one line of infantry, and in a second line, some squadrons of cavalry; at all events, only the chief post established in the centre has perhaps had some battalions in a second line.
3. A strategic reserve, to reinforce any point attacked has very seldom been kept in rear, because the extension of front made the line feel too weak already in all parts. On this account the support which a post attacked has received, has generally been furnished from other posts in the line not themselves attacked.
4. Even when the division of the forces has been relatively moderate, and the strength of each single post considerable, the principal resistance has been always confined to a local defence; and if once the enemy succeeded in wresting a post, it has been impossible to recover it by any supports afterwards arriving.
How much, according to this, may be expected from mountain defence, in what cases this means may be used, how far we can and may go in the extension and fractioning of the forces—these are all questions which theory must leave to the tact of the General. It is enough if it tells him what these means really are, and what rôle they can perform in the active operations of the Army.
A General who allows himself to be beaten in an extended mountain position deserves to be brought before a court-martial.