On War

by Carl von Clausewitz

In this manner every mountain system, as far as a certain distance, exercises a very great influence over the lower and more level country adjacent to it. Whether this influence shall take effect momentarily, for instance in a battle (as at Maltsch on the Rhine, 1796) or only after some time upon the lines of communication, depends on the local relations; whether or not it shall be overcome through some decisive event happening in the valley or level country, depends on the relations of the armed forces to each other respectively.

Buonaparte, in 1805 and 1809, advanced upon Vienna without troubling himself much about the Tyrol; but Moreau had to leave Swabia in 1796, chiefly because he was not master of the more elevated parts of the country, and too many troops were required to watch them. In campaigns, in which there is an evenly balanced series of alternate successes on each side, we shall not expose ourselves to the constant disadvantage of the mountains remaining in possession of the enemy: we need, therefore, only endeavour to seize and retain possession of that portion of them which is required on account of the direction of the principal lines of our attack; this generally leads to the mountains being the arena of the separate minor combats which take place between forces on each side. But we must be careful of overrating the importance of this circumstance, and being led to consider a mountain-chain as the key to the whole in all cases, and its possession as the main point. When a victory is the object sought; then it is the principal, object; and if the victory is gained, other things can be regulated according to the paramount requirement of the situation.

3. Mountains considered in their aspect of a strategic barrier.

We must divide this subject under two heads.

The first is again that of a decisive battle. We can, for instance, consider the mountain chain as a river, that is, as a barrier with certain points of passage, which may afford us an opportunity of gaining a victory, because the enemy will be compelled by it to divide his forces in advancing, and is tied down to certain roads, which will enable us with our forces concentrated behind the mountains to fall upon fractions of his force. As the assailant on his march through the mountains, irrespective of all other considerations, cannot march in a single column because he would thus expose himself to the danger of getting engaged in a decisive battle with only one line of retreat, therefore, the defensive method recommends itself certainly on substantial grounds. But as the conception of mountains and their outlets is very undefined, the question of adopting this plan depends entirely on the nature of the country itself, and it can only be pointed out as possible whilst it must also be considered as attended with two disadvantages, the first is, that if the enemy receives a severe blow, he soon finds shelter in the mountains; the second is, that he is in possession of the higher ground, which, although not decisive, must still always be regarded as a disadvantage for the pursuer.

We know of no battle given under such circumstances unless the battle with Alvinzi in 1796 can be so classed. But that the case may occur is plain from Buonaparte’s passage of the Alps in the year 1800, when Melas might and should have fallen on him with his whole force before he had united his columns.

The second influence which mountains may have as a barrier is that which they have upon the lines of communication if they cross those lines. Without taking into account what may be done by erecting forts at the points of passage and by arming the people, the bad roads in mountains at certain seasons of the year may of themselves alone prove at once destructive to an army; they have frequently compelled a retreat after having first sucked all the marrow and blood out of the army. If, in addition, troops of active partisans hover round, or there is a national rising to add to the difficulties, then the enemy’s army is obliged to make large detachments, and at last driven to form strong posts in the mountains and thus gets engaged in one of the most disadvantageous situations that can be in an offensive war.

4. Mountains in their relation to the provisioning of an army.

This is a very simple subject, easy to understand. The opportunity to make the best use of them in this respect is when the assailant is either obliged to remain in the mountains, or at least to leave them close in his rear.

These considerations on the defence of mountains, which, in the main, embrace all mountain warfare, and, by their reflection, throw also the necessary light on offensive war, must not be deemed incorrect or impracticable because we can neither make plains out of mountains, nor hills out of plains, and the choice of a theatre of war is determined by so many other things that it appears as if there was little margin left for considerations of this kind. In affairs of magnitude it will be found that this margin is not so small. If it is a question of the disposition and effective employment of the principal force, and that, even in the moment of a decisive battle, by a few marches more to the front or rear an army can be brought out of mountain ground into the level country, then a resolute concentration of the chief masses in the plain will neutralise the adjoining mountains.