On War

by Carl von Clausewitz

The second point on which we have to speak, is the removal of the means of crossing.—On the river itself the thing is no easy matter, at least requires considerable time; but on the affluents which fall into the river, particularly those on the enemy’s side, the difficulties are almost insurmountable, as these branch rivers are generally already in the hands of the enemy. For that reason it is important to close the mouths of such rivers by fortifications.

As the equipment for crossing rivers which an enemy brings with him, that is his pontoons, are rarely sufficient for the passage of great rivers, much depends on the means to be found on the river itself, its affluents, and in the great towns adjacent, and lastly, on the timber for building boats and rafts in forests near the river. There are cases in which all these circumstances are so unfavourable, that the crossing of a river is by that means almost an impossibility.

Lastly, the fortresses, which lie on both sides, or on the enemy’s side of the river, serve both to prevent any crossing at any points near them, up or down the river, and as a means of closing the mouths of affluents, as well as to receive immediately all craft or boats which may be seized.

So much as to the direct defence of a river, on the supposition that it is one containing a great volume of water. If a deep valley with precipitous sides or marshy banks, are added to the barrier of the river itself, then the difficulty of passing and the strength of the defence are certainly increased; but the volume of water is not made up for by such obstacles, for they constitute no absolute severance of the country, which is an indispensable condition of direct defence.

If we are asked what rôle such a direct river defence can play in the strategic plan of the campaign, we must admit that it can never lead to a decisive victory, partly because the object is not to let the enemy pass over to our side at all, or to crush the first mass of any size which passes; partly because the river prevents our being able to convert the advantages gained into a decisive victory by sallying forth in force.

On the other hand, the defence of a river in this way may produce a great gain of time, which is generally all important for the defensive. The collecting the means of crossing, takes up often much time; if several attempts fail a good deal more time is gained. If the enemy, on account of the river, gives his forces an entirely different direction, then still further advantages may be gained by that means. Lastly, whenever the enemy is not in downright earnest about advancing, a river will occasion a stoppage in his movements and thereby afford a durable protection to the country.

A direct defence of a river, therefore, when the masses of troops engaged are considerable, the river large, and other circumstances favourable, may be regarded as a very good defensive means, and may yield results to which commanders in modern times (influenced only by the thought of unfortunate attempts to defend rivers, which failed from insufficient means), have paid too little attention. For if, in accordance with the supposition just made (which may easily be realized in connection with such rivers as the Rhine or the Danube), an efficient defence of 24 miles of river is possible by 60,000 men in face of a very considerably superior force, we may well say that such a result deserves consideration.

We say, in opposition to a considerably superior force, and must again recur to that point. According to the theory we have propounded, all depends on the means of crossing, and nothing on the numerical strength of the force seeking to cross, always supposing it is not less than the force which defends the river. This appears very extraordinary, and yet it is true. But we must take care not to forget that most defences of rivers, or, more properly speaking, the whole, have no absolute points d’appui, therefore, may be turned, and this turning movement will be very much easier if the enemy has very superior numbers.

If now we reflect that such a direct defence of a river, even if overcome by the enemy, is by no means to be compared to a lost battle, and can still less lead to a complete defeat, since only a part of our force has been engaged, and the enemy, detained by the tedious crossing over of his troops on a single bridge, cannot immediately follow up his victory, we shall be the less disposed to despise this means of defence.

In all the practical affairs of human life it is important to hit the right point; and so also, in the defence of a river, it makes a great difference whether we rightly appreciate our situation in all its relations; an apparently insignificant circumstance may essentially alter the case, and make a measure which is wise and effective in one instance, a disastrous mistake in another. This difficulty of forming a right judgment and of avoiding the notion that “a river is a river” is perhaps greater here than anywhere else, therefore we must especially guard against false applications and interpretations; but having done so, we have also no hesitation in plainly declaring that we do not think it worth while to listen to the cry of those who, under the influence of some vague feeling, and without any fixed idea, expect everything from attack and movement, and think they see the most true picture of war in a hussar at full gallop brandishing his sword over his head.