On War

by Carl von Clausewitz

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 fire-arms, 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 detatchment, 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, etc., etc. 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 ten 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 fire-arms 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 defensive 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.