Front Page Titles (by Subject) CHAPTER XVIII: DEFENCE OF STREAMS AND RIVERS - On War, vol. 2
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CHAPTER XVIII: DEFENCE OF STREAMS AND RIVERS - Carl von Clausewitz, On War, vol. 2 
On War, trans. Col. J.J. Graham. New and Revised edition with Introduction and Notes by Col. F.N. Maude, in Three Volumes (London: Kegan Paul, Trench, Trubner & C., 1918). Vol. 2.
Part of: On War
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DEFENCE OF STREAMS AND RIVERS
Streams and large rivers, in so far as we speak of their defence, belong, like mountains, to the category of strategic barriers. But they differ from mountains in two respects. The one concerns their relative, the other their absolute defence.
Like mountains, they strengthen the relative defence; but one of their peculiarities is, that they are like implements of hard and brittle metal, they either stand every blow without bending, or their defence breaks and then ends altogether. If the river is very large, and the other conditions are favourable, then the passage may be absolutely impossible. But if the defence of any river is forced at one point, then there cannot be, as in mountain warfare, a persistent defence afterwards; the affair is finished with that one act, unless that the river itself runs between mountains.
The other peculiarity of rivers in relation to War is, that in many cases they admit of very good, and in general of better combinations than mountains for a decisive battle.
Both again have this property in common, that they are dangerous and seductive objects which have often led to false measures, and placed Generals in awkward situations. We shall notice these results in examining more closely the defence of rivers.
Although history is rather bare in examples of rivers defended with success, and therefore the opinion is justified that rivers and streams are no such formidable barriers as was once supposed, when an absolute defensive system seized all means of strengthening itself which the country offered, still the influence which they exercise to the advantage of the battle, as well as of the defence of a country, cannot be denied.
In order to look over the subject in a connected form, we shall specify the different points of view from which we propose to examine it.
First and foremost, the strategic results which streams and rivers produce through their defence, must be distinguished from the influence which they have on the defence of a country, even when not themselves specially defended.
Further, the defence itself may take three different forms:—
1. An absolute defence with the main body.
2. A mere demonstration of resistance.
3. A relative resistance by subordinate bodies of troops, such as outposts, covering lines, flanking detachments &c.
Lastly, we must distinguish three different degrees or kinds of defence, in each of its forms, namely—
1. A direct defence by opposing the passage.
2. A rather indirect one, by which the river and its valley are only used as a means towards a better combination for the battle.
3. A completely direct one, by holding an unassailable position on the enemy’s side of the river.
We shall subdivide our observations, in conformity with these three degrees, and after we have made ourselves acquainted with each of them in its relation to the first, which is the most important of the forms, we shall then proceed to do the same in respect to their relations to the other two. Therefore, first, the direct defence, that is, such a defence as is to prevent the passage of the enemy’s Army itself.
This can only come into the question in relation to large rivers, that is, great bodies of water.
The combinations of space, time, and force, which require to be looked into as elements of this theory of defence, make the subject somewhat complicated, so that it is not easy to gain a sure point from which to commence. The following is the result at which every one will arrive on full consideration.
The time required to build a bridge determines the distance from each other at which the detachments charged with the defence of the river should be posted. If we divide the whole length of the line of defence by this distance, we get the number of bodies required for the defence; if with that number we divide the mass of troops disposable, we shall get the strength of each detachment. If we now compare the strength of each single body with the number of troops which the enemy, by using all the means in his power, can pass over during the construction of his bridge, we shall be able to judge how far we can expect a successful resistance. For we can only assume the forcing of the passage to be impossible when the defender is able to attack the troops passed over with a considerable numerical superiority, say the double, before the bridge is completed. An illustration will make this plain.
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 twenty miles, therefore every forty miles 20,000 men would be required, which would make 60,000 for the defence of a length of one hundred and twenty 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 men 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 detachments as proposed by the present theory, must be posted close to the river, and each detachment concentrated in itself. 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 bodies thus placed than by a mere chain of posts, more particularly as the Commanders are all close at hand.—Each of these bodies 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 troops stationed close to the 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.
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 realised in connection with such rivers as the Rhine or the Danube), an efficient defence of one hundred and twenty 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.
Such ideas and feelings are not always all that is required (we shall only instance here the once famous dictator Wedel, at Zullichau, 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 body of 10,000 on the same length would not be able to oppose the passage of an equal number of 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 detachments 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, &c., &c., 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.
Its defect (the point on which the defender is very apt to err) is the over-extension of the defending force. It is so natural in such a case to be drawn on from one point of passage to another, and to miss the right point where we ought to stop; but then, if we do not succeed in fighting with the whole Army united, we miss the intended effect; a defeat in battle, the necessity of retreat, confusion in many ways and losses reduce the Army nearly to ruin, even although the resistance has not been pushed to an extremity.
In saying that the defensive, under the above conditions, should not extend his forces widely, that he should be in any case able to assemble all his forces on the evening of the day on which the enemy passes, enough is said, and it may stand in place of all combinations of time, power, and space, things which, in this case, must depend on many local points.
The battle to which these circumstances lead must have a special character—that of the greatest impetuosity on the side of the defender. The feigned passages by which the enemy will keep him for some time in uncertainty—will, in general, prevent his discovering the real point of crossing a moment too soon. The peculiar advantages of the situation of the defender consist in the disadvantageous situation of the enemy’s troops just immediately in his front; if other Corps, having passed at other points, menace his flank, he cannot, as in a defensive battle, counteract such movements by vigorous blows from his rear, for that would be to sacrifice the above-mentioned advantage of his situation; he must, therefore, decide the affair in his front before such other Corps can arrive and become dangerous, that is, he must attack what he has before him as swiftly and vigorously as possible, and decide all by its defeat.
But the object of this form of river defence can never be the repulse of a very greatly superior force, as is conceivable in the direct defence of a large river; for as a rule we have really to deal with the bulk of the enemy’s force, and although we do so under favourable circumstances, still it is easy to see the relation between the forces must soon be felt.
This is the nature of the defence of rivers of a moderate size and deep valleys when the principal masses of the Armies are concerned, for in respect to them the considerable resistance which can be offered on the ridges or scarps of the valley stands no comparison with the disadvantages of a scattered position, and to them a decisive victory is a matter of necessity. But if nothing more is wanted but the reinforcement of a secondary line of defence which is intended to hold out for a short time, and which can calculate on support, then certainly a direct defence of the scarps of the valley, or even of the river bank, may be made; and although the same advantages are not to be expected here as in mountain positions, still the resistance will always last longer than in an ordinary country. Only one circumstance makes this measure very dangerous, if not impossible: it is when the river has many windings and sharp turnings, which is just what is often the case when a river runs in a deep valley. Only look at the course of the Mosel. In a case of its defence, the Corps in advance on the salients of the bends would almost inevitably be lost in the event of a retreat.
That a great river allows the same defensive means, the same form of defence, which we have pointed out as best suited for rivers of a moderate size, in connection with the mass of an Army, and also under much more favourable circumstances, is plain of itself. It will come into use more especially when the point with the defender is to gain a decisive victory (Aspern).
The case of an Army drawn up with its front close on a river, or stream, or deep valley, in order by that means to command a tactical obstacle to the approach to its position, or to strengthen its front, is quite a different one, the detailed examination of which belongs to tactics. Of the effect of this we shall only say this much, that it is founded on a delusion.—If the cleft in the ground is very considerable, the front of the position becomes absolutely unassailable. Now, as there is no more difficulty in passing round such a position than any other, it is just the same as if the defender had himself gone out of the way of the assailant, yet that could hardly be the object of the position. A position of this kind can, therefore, only be advisable when, as a consequence of its position, it threatens the communications of the assailant, so that every deviation by him from the direct road is fraught with consequences altogether too serious to be risked.
In this second form of defence, feigned passages are much more dangerous, for the assailant can make them more easily, while, on the other hand, the proposition for the defender is, to assemble his whole Army at the right point. But the defender is certainly not quite so much limited for time here, because the advantage of his situation lasts until the assailant has massed his whole force, and made himself master of several crossings; moreover, also, the simulated attack has not the same degree of effect here as in the defence of a cordon, where all must be held, and where, therefore, in the application of the reserve, it is not merely a question, as in our proposition, where the enemy has his principal force, but the much more difficult one, Which is the point he will first seek to force?
With respect to both forms of defence of large and small rivers, we must observe generally, that if they are undertaken in the haste and confusion of a retreat, without preparation, without the removal of all means of passage, and without an exact knowledge of the country, they cannot certainly fulfil what has been here supposed; in most such cases, nothing of the kind is to be calculated upon; and therefore it will be always a great error for an Army to divide itself over extended positions.
As everything usually miscarries in War, if it is not done upon clear convictions and with the whole will and energy, so a river defence will generally end badly when it is only resorted to because we have not the heart to meet the enemy in the open field, and hope that the broad river or the deep valley will stop him. When that is the case, there is so little confidence in the actual situation that both the General and his Army are usually filled with anxious forebodings, which are almost sure to be realised quick enough. A battle in the open field does not suppose a perfectly equal state of circumstances beforehand, like a duel; and the defender who does not know how to gain for himself any advantages, either through the special nature of the defence, through rapid marches, or by knowledge of the country and freedom of movement, is one whom nothing can save, and least of all will a river or its valley be able to help him.
The third form of defence—by a strong position taken up on the enemy’s side of the river—founds its efficacy on the danger in which it places the enemy of having his communications cut by the river, and being thus limited to a few bridges only. It follows, as a matter of course, that we are only speaking of great rivers with a great volume of water, as these alone can lead to such results, whilst a river which is merely in a deep ravine usually affords such a number of passages that all danger of the above disappears.
But the position of the defensive must be very strong, almost unassailable; otherwise he would just meet the enemy half way, and give up his advantages. But if it is of such strength that the enemy resolves not to attack it, he will, under certain circumstances, be confined thereby to the same bank with the defender. If the assailant crosses, he exposes his communications; but certainly, at the same time, he threatens ours. Here, as in all cases in which one Army passes by another, the great point is, whose communications, by their number, situation, and other circumstances, are the best secured, and which has also, in other respects, most to lose, therefore can be outbid by his opponent; lastly, which possesses still in his Army the most power of victory upon which he can depend in an extreme case. The influence of the river merely amounts to this, that it augments the danger of such a movement for both parties, as both are dependent on bridges. Now, in so far as we can assume that, according to the usual course of things, the passage of the defender, as well as of his depôts of all kinds, are better secured by fortresses than those of the offensive, in so far is such a defence conceivable, and one which might be substituted for the direct defence when circumstances are not favourable to that form. Certainly then the river is not defended by the Army, nor the Army by the river, but by the connection between the two the country is defended, which is the main point.
At the same time it must be granted that this mode of defence, without a decisive blow, and resembling the state of tension of two electric currents, of which the atmospheres only are as yet in contact, cannot stop any very powerful impulsive force. It might be applicable against even a great superiority of force on the side of the enemy, if their Army is commanded by a cautious General, wanting in decision, and never disposed to push forward with energy; it might also answer when a kind of oscillation towards equality between the contending forces has previously arisen, and nothing but small advantages are looked for on either side. But if we have to deal with superior forces, led by a bold General, we are upon a dangerous course, very close to an abyss.
This form of defence looks so bold, and at the same time so scientific, that it might be called the elegant; but as elegance easily merges into folly, and as it is not so easily excused in War as in society, therefore we have had as yet few instances of this elegant art. From this third mode a special means of assistance for the first two forms is developed, that is, by the permanent occupation of a bridge and a tête du pont to keep up a constant threat of crossing.
Besides the object of an absolute defence with the main body, each of the three modes of defence may also have that of a feigned defence.
This show of a resistance, which it is not intended really to offer, is an act which is combined with many other measures, and fundamentally with every position which is anything more than a camp of route; but the feigned defence of a great river becomes a complete stratagem in this way, that it is necessary to adopt actually more or less a number of measures of detail, and that its action is usually on a greater scale and of longer duration than that of any other; for the act of passing a great river in sight of an Army is always an important step for the assailant, one over which he often ponders long, or which he postpones to a more favourable moment.
For such a feigned defence it is therefore requisite that the main Army should divide and post itself along the river (much in the same manner as for a real defence); but as the intention of a mere demonstration shows that circumstances are not favourable enough for a real defence, therefore, from that measure as it always occasions a more or less extended and scattered disposition, the danger of serious loss may very easily arise if the detachments should get engaged in a real resistance, even if not carried to an extremity; it would then be in the true sense a half measure. In a demonstration of defence, therefore, arrangement must be made for a sure concentration of the Army at a point considerably (perhaps several days’ march) in rear, and the defence should not be carried beyond what is consistent with this arrangement.
In order to make our views plainer, and to show the importance of such a defensive demonstration, let us refer to the end of the campaign of 1813. Buonaparte repassed the Rhine with forty or fifty thousand men. To attempt to defend this river with such a force at all points where the Allies, according to the direction of their forces, might easily pass, that is, between Manheim and Nimeguen, would have been to attempt an impossibility. The only idea which Buonaparte could therefore entertain was to offer his first real resistance somewhere on the French Meuse, where he could make his appearance with his Army in some measure reinforced. Had he at once withdrawn his forces to that point, the Allies would have followed close at his heels; had he placed his Army in cantonments for rest behind the Rhine, the same thing must have taken place almost as soon, for at the least show of desponding caution on his part, the Allies would have sent over swarms of Cossacks and other light troops in pursuit, and, if that measure produced good results, other Corps would have followed. The French Corps had therefore nothing for it but to take steps to defend the Rhine in earnest. As Buonaparte could foresee that this defence must end in nothing whenever the Allies seriously undertook to cross the river, it may therefore be regarded in the light of a mere demonstration, in which the French Corps incurred hardly any danger, as their point of concentration lay on the Upper Moselle. Only Macdonald, who, as is known, was at Nimeguen with twenty thousand men, committed a mistake in deferring his retreat till fairly compelled to retire, for this delay prevented his joining Buonaparte before the battle of Brienne, as the retreat was not forced on him until after the arrival of Winzingerode’s Corps in January. This defensive demonstration on the Rhine, therefore, produced the result of checking the Allies in their advance, and induced them to postpone the crossing of the river until their reinforcements arrived, which did not take place for six weeks. These six weeks were of infinite value to Buonaparte. Without this defensive demonstration on the Rhine, Paris would have become the next immediate object after the victory of Leipsic, and it would have been impossible for the French to have given battle on that side of their capital.
In a river defence of the second class, therefore, in that of rivers of a smaller size, such demonstrations may also be used, but they will generally be less effectual, because mere attempts to cross are in such a case easier, and therefore the spell is sooner broken.
In the third kind of river defence, a demonstration would in all probability be still less effectual, and produce no more result than that of the occupation of any other temporary position.
Lastly, the two first forms of defence are very well suited to give a chain of outposts, or any other defensive line (cordon) established for a secondary object, or to a corps of observation, much greater and more reliable strength than it would have without the river. In all these cases the question is limited to a relative resistance and that must naturally be considerably strengthened by such a great natural obstacle. At the same time, we must not think only of the relative quantity of time gained by the resistance in fight in a case of this sort, but also of the many anxieties which such undertakings usually excite in the mind of the enemy, and which in ninety-nine cases out of a hundred lead to his giving up his plans if not urged or pressed by necessity.