Front Page Titles (by Subject) CHAPTER XV: DEFENCE OF MOUNTAINS - On War, vol. 2
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CHAPTER XV: DEFENCE OF MOUNTAINS - 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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DEFENCE OF MOUNTAINS
The influence of mountains on the conduct of War is very great; the subject, therefore, is very important for theory. As this influence introduces into action a retarding principle, it belongs chiefly to the defensive. We shall therefore discuss it here in a wider sense than that conveyed by the simple conception, defence of mountains. As we have discovered in our consideration of the subject results which run counter to general opinion in many points, we shall therefore be obliged to enter into rather an elaborate analysis of it.
We shall first examine the tactical nature of the subject, in order to gain the point where it connects itself with strategy.
The endless difficulty attending the march of large columns on mountain roads, the extraordinary strength which a small post obtains by a steep scarp covering its front, and by ravines right and left supporting its flanks, are unquestionably the principal causes why such efficacy and strength are universally attributed to the defence of mountains, so that nothing but the peculiarities in armament and tactics at certain periods has prevented large masses of combatants from engaging in it.
When a column, winding like a serpent, toils its way through narrow ravines up to the top of a mountain, and passes over it at a snail’s pace, artillery and train-drivers with oaths and shouts, flogging their over-driven cattle through the narrow rugged roads, each broken waggon has to be got out of the way with indescribable trouble, whilst all behind are detained, cursing and blaspheming, every one then thinks to himself, Now if the enemy should appear with only a few hundred men, he might disperse the whole. From this has originated the expression used by historical writers, when they describe a narrow pass as a place where “a handful of men might keep an army in check.” At the same time, every one who has had any experience in War knows, or ought to know, that such a march through mountains has little or nothing in common with the attack of these same mountains, and that therefore to infer from the difficulty of marching through mountains that the difficulty of attacking them must be much greater is a false conclusion.
It is natural enough that an inexperienced person should thus argue, and it is almost as natural that the Art of War itself for a certain time should have been entangled in the same error, for the fact which it related to was almost as new at that time to those accustomed to War as to the uninitiated. Before the Thirty Years’ War, owing to the deep order of battle, the numerous cavalry, the rude firearms, and other peculiarities, it was quite unusual to make use of formidable obstacles of ground in War, and a formal defence of mountains, at least by regular troops, was almost impossible. It was not until a more extended order of battle was introduced, and that infantry and their arms became the chief part of an Army, that the use which might be made of hills and valleys occurred to men’s minds. But it was not until a hundred years afterwards, or about the middle of the eighteenth century, that the idea became fully developed.
The second circumstance, namely, the great defensive capability which might be given to a small post planted on a point difficult of access, was still more suited to lead to an exaggerated idea of the strength of mountain defences. The opinion arose that it was only necessary to multiply such a post by a certain number to make an Army out of a battalion, a chain of mountains out of a mountain.
It is undeniable that a small post acquires an extraordinary strength by selecting a good position in a mountainous country. A small detachment, which would be driven off in the level country by a couple of squadrons, and think itself lucky to save itself from rout or capture by a hasty retreat, can in the mountains stand up before a whole Army, and, as one might say, with a kind of tactical effrontery exact the military honour of a regular attack, of having its flank turned, &c., &c. How it obtains this defensive power, by obstacles to approach, points d’appui for its flanks, and new positions which it finds on its retreat, is a subject for tactics to explain; we accept it as an established fact.
It was very natural to believe that a number of such posts placed in a line would give a very strong, almost unassailable front, and all that remained to be done was to prevent the position from being turned by extending it right and left until either flank-supports were met with commensurate with the importance of the whole, or until the extent of the position itself gave security against turning movements. A mountainous country specially invites such a course by presenting such a succession of defensive positions, each one apparently better than another, that one does not know where to stop; and therefore it ended in all and every approach to the mountains within a certain distance being guarded, with a view to defence, and ten or fifteen single posts, thus spread over a space of about fifty miles or more, were supposed to bid defiance to that odious turning movement. Now as the connection between these posts was considered sufficiently secure by the intervening spaces, being ground of an impassable nature (columns at that time not being able to quit the regular roads), it was thought a wall of brass was thus presented to the enemy. As an extra precaution, a few battalions, some horse artillery, and a dozen squadrons of cavalry, formed a reserve to provide against the event of the line being unexpectedly burst through at any point.
No one will deny that the prevalence of this idea is shown by history, and it is not certain that at this day we are completely emancipated from these errors.
The course of improvement in tactics since the Middle Ages, with the ever increasing strength of Armies, likewise contributed to bring mountainous districts in this sense more within the scope of military action.
The chief characteristic of mountain defence is its complete passivity; in this light the tendency towards the defence of mountains was very natural before Armies attained to their present capability of movement. But Armies were constantly becoming greater, and on account of the effect of firearms began to extend more and more into long thin lines connected with a great deal of art, and on that account very difficult, often almost impossible, to move. To dispose, in order of battle, such an artistic machine, was often half a day’s work, and half the battle; and almost all which is now attended to in the preliminary plan of the battle was included in this first disposition or drawing up. After this work was done it was therefore difficult to make any modifications to suit new circumstances which might spring up; from this it followed that the assailant, being the last to form his line of battle, naturally adapted it to the order of battle adopted by the enemy, without the latter being able in turn to modify his in accordance. The attack thus acquired a general superiority, and the defence had no other means of reinstating the balance than that of seeking protection from the impediments of ground, and for this nothing was so favourable in general as mountainous ground. Thus it became an object to couple, as it were, the Army with a formidable obstacle of ground, and the two united then made common cause. The battalion defended the mountain, and the mountain the battalion; so the passive defence through the aid of mountainous ground became highly efficacious, and there was no other evil in the thing itself except that it entailed a greater loss of freedom of movement, but of that quality they did not understand the particular use at that time.
When two antagonistic systems act upon each other, the exposed, that is, the weak point on the one side always draws upon itself the blows from the other side. If the defensive becomes fixed, and as it were, spell-bound in posts, which are in themselves strong, and can not be taken, the aggressor then becomes bold in turning movements, because he has no apprehension about his own flanks. This is what took place—The turning, as it was called, soon became the order of the day: to counteract this, positions were extended more and more; they were thus weakened in front, and the offensive suddenly turned upon that part: instead of trying to outflank by extending, the assailant now concentrated his masses for attack at some one point, and the line was broken. This is nearly what took place in regard to mountain defences according to the latest modern history.
The offensive had thus again gained a preponderance through the greater mobility of troops; and it was only through the same means that the defence could seek for help. But mountainous ground by its nature is opposed to mobility, and thus the whole theory of mountain defence experienced, if we may use the expression, a defeat like that which the Armies engaged in it in the Revolutionary War so often suffered.
But that we may not reject the good with the bad, and allow ourselves to be carried along by the stream of commonplace to assertions which, in actual experience, would be refuted a thousand times by the force of circumstances, we must distinguish the effects of mountain defence according to the nature of the cases.
The principal question to be decided here, and that which throws the greatest light over the whole subject is, whether the resistance which is intended by the defence of mountains is to be relative or absolute—whether it is only intended to last for a time, or is meant to end in a decisive victory. For a resistance of the first kind mountainous ground is in a high degree suitable, and introduces into it a very powerful element of strength; for one of the latter kind, on the contrary, it is in general not at all suitable, or only so in some special cases.
In mountains every movement is slower and more difficult, costs therefore more time, and more men as well, if within the sphere of danger. But the loss of the assailant in time and men is the standard by which the defensive resistance is measured. As long as the movement is all on the side of the offensive so long the defensive has a marked advantage; but as soon as the defensive resorts to this principle of movement also, that advantage ceases. Now from the nature of the thing, that is to say, on tactical grounds, a relative resistance allows of a much greater degree of passivity than one which is intended to lead to a decisive result, and it allows this passivity to be carried to an extreme, that is, to the end of the combat, which in the other case can never happen. The impeding element of mountain ground, which as a medium of greater density weakens all positive activity, is, therefore, completely suited to the passive defence.
We have already said that a small post acquires an extraordinary strength by the nature of the ground; but although this tactical result in general requires no further proof, we must add to what we have said some explanation. We must be careful here to draw a distinction between what is relatively and what is absolutely small. If a body of troops, let its size be what it may, isolates a portion of itself in a position, this portion may possibly be exposed to the attack of the whole body of the enemy’s troops, therefore of a superior force, in opposition to which it is itself small. There, as a rule, no absolute but only a relative defence can be the object. The smaller the post in relation to the whole body from which it is detached and in relation to the whole body of the enemy, the more this applies.
But a post also which is small in an absolute sense, that is, one which is not opposed by an enemy superior to itself, and which, therefore, may aspire to an absolute defence, a real victory, will be infinitely better off in mountains than a large Army, and can derive more advantage from the ground as we shall show further on.
Our conclusion, therefore, is, that a small post in mountains possesses great strength. How this may be of decisive utility in all cases which depend entirely on a relative defence is plain of itself; but will it be of the same decisive utility for the absolute defence by a whole Army? This is the question which we now propose to examine.
First of all we ask whether a front line composed of several posts has, as has hitherto been assumed, the same strength proportionally as each post singly. This is certainly not the case, and to suppose so would involve one of two errors.
In the first place, a country without roads is often confounded with one which is quite impassable. Where a column, or where artillery and cavalry cannot march, infantry may still, in general, be able to pass, and even artillery may often be brought there as well, for the movements made in a battle by excessive efforts of short duration are not to be judged of by the same scale as marches. The secure connection of the single posts with one another rests therefore on an illusion, and the flanks are in reality in danger.
Or next it is supposed, a line of small posts, which are very strong in front, are also equally strong on their flanks, because a ravine, a precipice, &c., &c., form excellent supports for a small post. But why are they so?—not because they make it impossible to turn the post, but because they cause the enemy an expenditure of time and of force, which gives scope for the effectual action of the post. The enemy who, in spite of the difficulty of the ground, wishes, and in fact is obliged, to turn such a post, because the front is unassailable requires, perhaps, half-a-day to execute his purpose, and cannot after all accomplish it without some loss of men. Now if such a post can be succoured, or if it is only designed to resist for a certain space of time, or lastly, if it is able to cope with the enemy, then the flank supports have done their part, and we may say the position had not only a strong front, but strong flanks as well. But it is not the same if it is a question of a line of posts, forming part of an extended mountain position. None of these three conditions are realised in that case. The enemy attacks one point with an overwhelming force, the support in rear is perhaps slight, and yet it is a question of absolute resistance. Under such circumstances the flank supports of such posts are worth nothing.
Upon a weak point like this the attack usually directs its blows. The assault with concentrated, and therefore very superior forces, upon a point in front, may certainly be met by a resistance, which is very violent as regards that point, but which is unimportant as regards the whole. After it is overcome, the line is pierced, and the object of the attack attained.
From this it follows that the relative resistance in mountain warfare is, in general, greater than in a level country, that it is comparatively greatest in small posts, and does not increase in the same measure as the masses increase.
Let us now turn to the real object of great battles generally—to the positive victory which may also be the object in the defence of mountains. If the whole mass, or the principal part of the force, is employed for that purpose, then the defence of mountains changes itself eo ipso into a defensive battle in the mountains. A battle, that is the application of all our powers to the destruction of the enemy is now the form, a victory the object of the combat. The defence of mountains which takes place in this combat, appears now a subordinate consideration, for it is no longer the object, it is only the means. Now in this view, how does the ground in mountains answer to the object?
The character of a defensive battle is a passive reaction in front, and an increased active reaction in rear; but for this the ground in mountains is a paralysing principle. There are two reasons for this: first, want of roads affording means of rapidly moving in all directions, from the rear towards the front, and even the sudden tactical attack is hampered by the unevenness of ground; secondly, a free view over the country, and the enemy’s movements is not to be had. The ground in mountains, therefore, ensures in this case to the enemy the same advantages which it gave to us in the front, and deadens all the better half of the resistance. To this is to be added a third objection, namely the danger of being cut off. Much as a mountainous country is favourable to a retreat, made under a pressure exerted along the whole front, and great as may be the loss of time to an enemy who makes a turning movement in such a country, still these again are only advantages in the case of a relative defence, advantages which have no connection with the decisive battle, the resistance to the last extremity. The resistance will last certainly somewhat longer, that is until the enemy has reached a point with his flank-columns which menaces or completely bars our retreat. Once he has gained such a point then relief is a thing hardly possible. No act of the offensive which we can make from the rear can drive him out again from the points which threaten us; no desperate assault with our whole mass can clear the passage which he blocks. Whoever thinks he discovers in this a contradiction, and believes that the advantages which the assailant has in mountain warfare, must also accrue to the defensive in an attempt to cut his way through, forgets the difference of circumstances. The corps which opposes the passage is not engaged in an absolute defence, a few hours’ resistance will probably be sufficient; it is, therefore, in the situation of a small post. Besides this, its opponent is no longer in full possession of all his fighting powers; he is thrown into disorder, wants ammunition, &c. Therefore, in any view, the chance of cutting through is small, and this is the danger that the defensive fears above all; this fear is at work even during the battle, and enervates every fibre of the struggling athlete. A nervous sensibility springs up on the flanks, and every small detachment which the aggressor makes a display of on any wooded eminence in our rear, is for him a new lever, helping on the victory.
These disadvantages will, for the most part, disappear, leaving all the advantages, if the defence of a mountain district consists in the concentrated disposition of the Army on an extensive mountain plateau. There we may imagine a very strong front; flanks very difficult of approach, and yet the most perfect freedom of movement, both within and in rear of the position. Such a position would be one of the strongest that there can be, but it is little more than an illusion, for although most mountains are more easily traversed along their crests than on their declivities, yet most plateaux of mountains are either too small for such a purpose, or they have no proper right to be called plateaux, and are so termed more in a geological, than in a geometrical sense.
For smaller bodies of troops, the disadvantages of a defensive position in mountains diminish as we have already remarked. The cause of this is, that such bodies take up less space, and require fewer roads for retreat, &c., &c. A single hill is not a mountain system, and has not the same disadvantages. The smaller the force, the more easily it can establish itself on a single ridge or hill, and the less will be the necessity for it to get entangled in the intricacies of countless steep mountain gorges.