On War

by Carl von Clausewitz

Such ideas and feelings are not always all that is required (we shall only instance here the once famous dictator Wedel, at Züllichau, in 1759); but the worst of all is that they are seldom durable, and they forsake the general at the last moment if great complex cases branching out into a thousand relations bear heavily upon him.

We therefore believe that a direct defence of a river with large bodies of troops, under favourable conditions, can lead to successful results if we content ourselves with a moderate negative: but this does not hold good in the case of smaller masses. Although 60,000 men on a certain length of river could prevent an army of 100,000 or more from passing, a corps of 10,000 on the same length would not be able to oppose the passage of a corps of 10,000 men, indeed, probably, not of one half that strength if such a body chose to run the risk of placing itself on the same side of the river with an enemy so much superior in numbers. The case is clear, as the means of passing do not alter.

We have as yet said little about feints or demonstrations of crossing, as they do not essentially come into consideration in the direct defence of a river, for partly such defence is not a question of concentration of the army at one point, but each corps has the defence of a portion of the river distinctly allotted to it; partly such simulated intentions of crossing are also very difficult under the circumstances we have supposed. If, for instance, the means of crossing in themselves are already limited, that is, not in such abundance as the assailant must desire to ensure the success of his undertaking, he will then hardly be able or willing to apply a large share to a mere demonstration: at all events the mass of troops to be passed over at the true point of crossing must be so much the less, and the defender gains again in time what through uncertainty he may have lost.

This direct defence, as a rule, seems only suitable to large rivers, and on the last half of their course.

The second form of defence is suitable for smaller rivers with deep valleys, often also for very unimportant ones. It consists in a position taken up further back from the river at such a distance that the enemy’s army may either be caught in detail after the passage (if it passes at several points at the same time) or if the passage is made by the whole at one point, then near the river, hemmed in upon one bridge and road. An army with the rear pressed close against a river or a deep valley, and confined to one line of retreat, is in a most disadvantageous position for battle; in the making proper use of this circumstance, consists precisely the most efficacious defence of rivers of moderate size, and running in deep valleys.

The disposition of an army in large corps close to a river which we consider the best in a direct defence, supposes that the enemy cannot pass the river unexpectedly and in great force, because otherwise, by making such a disposition, there would be great danger of being beaten in detail. If, therefore, the circumstances which favour the defence are not sufficiently advantageous, if the enemy has already in hand ample means of crossing, if the river has many islands or fords, if it is not broad enough, if we are too weak, etc., etc., then the idea of that method may be dismissed: the troops for the more secure connection with each other must be drawn back a little from the river, and all that then remains to do is to ensure the most rapid concentration possible upon that point where the enemy attempts to cross, so as to be able to attack him before he has gained so much ground that he has the command of several passages. In the present case the river or its valley must be watched and partially defended by a chain of outposts whilst the army is disposed in several corps at suitable points and at a certain distance (usually a few leagues) from the river.

The most difficult point lies here in the passage through the narrow way formed by the river and its valley. It is not now only the volume of water in the river with which we are concerned, but the whole of the defile, and, as a rule, a deep rocky valley is a greater impediment to pass than a river of considerable breadth. The difficulty of the march of a large body of troops through a long defile is in reality much greater than appears at first consideration. The time required is very considerable; and the danger that the enemy during the march may make himself master of the surrounding heights must cause disquietude. If the troops in front advance too far, they encounter the enemy too soon, and are in danger of being overpowered; if they remain near the point of passage then they fight in the worst situation. The passage across such an obstacle of ground with a view to measure strength with the enemy on the opposite side is, therefore, a bold undertaking, or it implies very superior numbers and great confidence in the commander.

Such a defensive line cannot certainly be extended to such a length as in the direct defence of a great river, for it is intended to fight with the whole force united, and the passages, however difficult, cannot be compared in that respect with those over a large river; it is, therefore, much easier for the enemy to make a turning movement against us. But at the same time, such a movement carries him out of his natural direction (for we suppose, as is plain in itself, that the valley crosses that direction at about right angles), and the disadvantageous effect of a confined line of retreat only disappears gradually, not at once, so that the defender will still always have some advantage over the advancing foe, although the latter is not caught exactly at the crisis of the passage, but by the detour he makes is enabled to get a little more room to move.

As we are not speaking of rivers in connection only with the mass of their waters, but have rather more in view the deep cleft or channel formed by their valleys, we must explain that under the term we do not mean any regular mountain gorge, because then all that has been said about mountains would be applicable. But, as every one knows, there are many level districts where the channels of even the smallest streams have deep and precipitous sides; and, besides these, such as have marshy banks, or whose banks are otherwise difficult of approach, belong to the same class.

Under these conditions, therefore, an army on the defensive, posted behind a large river or deep valley with steep sides, is in a very excellent position, and this sort of river defence is a strategic measure of the best kind.