On War

by Carl von Clausewitz

If the enemy requires twenty-four hours for the construction of a bridge, and if he can by other means only pass over 20,000 men in those twenty-four hours, whilst the defender within twelve hours can appear at any point whatever with 20,000 men, in such case the passage cannot be forced; for the defender will arrive when the enemy engaged in crossing has only passed over the half of 20,000. Now as in twelve hours, the time for conveying intelligence included, we can march four miles, therefore every eight miles 20,000 men would be required, which would make 60,000 for the defence of a length of twenty-four miles of river. These would be sufficient for the appearance of 20,000 men at any point, even if the enemy attempted the passage at two points at the same time; if at only one point twice 20,000 could be brought to oppose him at that single point.

Here, then, there are three circumstances exercising a decisive influence: (1) the breadth of the river; (2) the means of passage, for the two determine both the time required to construct the bridge, and the number of troops that can cross during the time the bridge is being built; (3) the strength of the defender’s army. The strength of the enemy’s force itself does not as yet come into consideration. According to this theory we may say that there is a point at which the possibility of crossing completely stops, and that no numerical superiority on the part of the enemy would enable him to force a passage.

This is the simple theory of the direct defence of a river, that is, of a defence intended to prevent the enemy from finishing his bridge and from making the passage itself; in this there is as yet no notice taken of the effect of demonstrations which the enemy may use. We shall now bring into consideration particulars in detail, and measures requisite for such a defence.

Setting aside, in the first place, geographical peculiarities, we have only to say that the corps as proposed by the present theory, must be posted close to the river, and each corps in itself concentrated. It must be close to the river, because every position further back lengthens unnecessarily and uselessly the distance to be gone over to any point menaced; for as the waters of the river give security against any important movement on the part of the enemy, a reserve in rear is not required, as it is for an ordinary line of defence, where there is no river in front. Besides, the roads running parallel to and near a river up and down, are generally better than transverse roads from the interior leading to any particular points on the river. Lastly, the river is unquestionably better watched by corps thus placed than by a mere chain of posts, more particularly as the commanders are all close at hand.—Each of these corps must be concentrated in itself, because otherwise all the calculation as to time would require alteration. He who knows the loss of time in effecting a concentration, will easily comprehend that just in this concentrated position lies the great efficacy of the defence. No doubt, at first sight, it is very tempting to make the crossing, even in boats, impossible for the enemy by a line of posts; but with a few exceptions of points, specially favourable for crossing, such a measure would be extremely prejudicial. To say nothing of the objection that the enemy can generally drive off such a post by bringing a superior force to bear on it from the opposite side, it is, as a rule, a waste of strength, that is to say, the most that can be obtained by any such post, is to compel the enemy to choose another point of passage. If, therefore, we are not so strong that we can treat and defend the river like a ditch of a fortress, a case for which no new precept is required, such a method of directly defending the bank of a river leads necessarily away from the proposed object. Besides these general principles for positions, we have to consider—first, the examination of the special peculiarities of the river; second, the removal of all means of passage; third, the influence of any fortresses situated on the river.

A river, considered as a line of defence, must have at the extremities of the line, right and left, points d’appui, such as, for instance, the sea, or a neutral territory; or there must be other causes which make it impracticable for the enemy to turn the line of defence by crossing beyond its extremities. Now, as neither such flank supports nor such impediments are to be found, unless at considerable distances, we see at once that the defence of a river must embrace a considerable portion of its length, and that, therefore, the possibility of a defence by placing a large body of troops behind a relatively short length of the river vanishes from the class of possible facts (to which we must always confine ourselves). We say a relatively short length of the river, by which we mean a length which does not very much exceed that which the same number of troops would usually occupy on an ordinary position in line without a river. Such cases, we say, do not occur, and every direct defence of a river always becomes a kind of cordon system, at least as far as regards the extension of the troops, and therefore is not at all adapted to oppose a turning movement on the part of the enemy in the same manner which is natural to an army in a concentrated position. Where, therefore, such turning movement is possible, the direct defence of the river, however promising its results in other respects, is a measure in the highest degree dangerous.

Now, as regards the portion of the river between its extreme points, of course we may suppose that all points within that portion are not equally well suited for crossing. This subject admits of being somewhat more precisely determined in the abstract, but not positively fixed, for the very smallest local peculiarity often decides more than all which looks great and important in books. Besides, it is wholly unnecessary to lay down any rules on this subject, for the appearance of the river, and the information to be obtained from those residing near it, will always amply suffice, without referring back to books.

As matters of detail, we may observe that roads leading down upon a river, its affluents, the great towns through which it passes, and lastly above all, its islands, generally favour a passage the most; that on the other hand, the elevation of one bank over another, and the bend in the course of the river at the point of passage, which usually act such a prominent rôle in books, are seldom of any consequence. The reason of this is, that the presumed influence of these two things rests on the limited idea of an absolute defence of the river bank—a case which seldom or never happens in connection with great rivers.

Now, whatever may be the nature of the circumstances which make it easier to cross a river at particular points, they must have an influence on the position of the troops, and modify the general geometrical law; but it is not advisable to deviate too far from that law, relying on the difficulties of the passage at many points. The enemy would choose exactly those spots which are the least favourable by nature for crossing, if he knew that these are the points where there is the least likelihood of meeting us.

In any case the strongest possible occupation of islands is a measure to be recommended, because a serious attack on an island indicates in the surest way the intended point of passage.

As the corps stationed close to a river must be able to move either up or down along its banks according as circumstances require, therefore if there is no road parallel to the river, one of the most essential preparatory measures for the defence of the river is to put the nearest small roads running in a parallel direction into suitable order, and to construct such short roads of connection as may be necessary.