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CHAPTER XX - 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 SWAMPS
Very large wide swamps, such as the Bourtang Moor in North Germany, are so uncommon that it is not worth while to lose time over them; but we must not forget that certain lowlands and marshy banks of small rivers are more common, and form very considerable obstacles of ground which may be, and often have been, used for defensive purposes.
Measures for their defence are certainly very like those for the defence of rivers, at the same time there are some peculiarities to be specially noticed. The first and principal one is, that a marsh which except on the causeway is impracticable for infantry is much more difficult to cross than any river; for, in the first place, a causeway is not so soon built as a bridge; secondly, there are no means at hand by which the troops to cover the construction of the dyke or causeway can be sent across. No one would begin to build a bridge without using some of the boats to send over an advance guard in the first instance; but in the case of a morass no similar assistance can be employed; the easiest way to make a crossing for infantry over a morass is by means of planks, but when the morass is of some width, this is a much more tedious process than the crossing of the first boats on a river. If now, besides, there is in the middle of the morass a river which cannot be passed without a bridge, the crossing of the first detachment of troops becomes a still more difficult affair, for although single passengers may get across on boards, the heavy material required for bridge building cannot be so transported. This difficulty on many occasions may be insurmountable.
A second peculiarity of a swamp is, that the means used to cross cannot be completely removed like those used for passing a river; bridges may be broken, or so completely destroyed that they can never be used again; the most that can be done with dykes is to cut them, which is not doing much. If there is a river in the middle, the bridge can of course be taken away, but the whole passage will not by that means be destroyed in the same degree as that of a large river by the destruction of a bridge. The natural consequence is that dykes which exist must always be occupied in force and strenuously defended if we desire to derive any general advantage from the morass.
On the one hand, therefore, we are compelled to adopt a local defence, and on the other, such a defence is favoured by the difficulty of passing at other parts. From these two peculiarities the result is, that the defence of a swamp must be more local and passive than that of a river.
It follows from this that we must be stronger in a relative degree than in the direct defence of a river, consequently that the line of defence must not be of great length, especially in cultivated countries, where the number of passages, even under the most favourable circumstances for defence, is still very great.
In this respect, therefore, swamps are inferior to great rivers, and this is a point of great importance, for all local defence is illusory and dangerous to an extreme. But if we reflect that such swamps and low grounds generally have a breadth with which that of the largest rivers in Europe bears no comparison, and that consequently a post stationed for the defence of a passage is never in danger of being overpowered by the fire from the other side, that the effects of its own fire over a long narrow dyke is greatly increased, and that the time required to pass such a defile, perhaps miles long, is much greater than would suffice to pass an ordinary bridge: if we consider all this, we must admit that such low lands and morasses, if means of crossing are not too numerous, belong to the strongest lines of defence which can be formed.
An indirect defence, such as we made ourselves acquainted with in the case of streams and rivers, in which obstacles of ground are made use of to bring on a great battle under advantageous circumstances, is generally quite as applicable to morasses.
The third method of a river-defence by means of a position on the enemy’s side would be too hazardous on account of the toilsome nature of the crossing.
It is extremely dangerous to venture on the defence of such morasses, soft meadows, bogs, &c., as are not quite impassable beyond the dykes. One single line of crossing discovered by the enemy is sufficient to pierce the whole line of defence which, in case of a serious resistance, is always attended with great loss to the defender.
We have still to consider inundations. As defensive means and also as phenomena in the natural world they have unquestionably the nearest resemblance to morasses.
They are not common certainly; perhaps Holland is the only country in Europe where they constitute a phenomenon which makes them worth notice in connection with our object; but just that country, on account of the remarkable campaigns of 1672 and 1787, as well as on account of its important relation in itself to both France and Germany, obliges us to devote some consideration to this matter.
The character of these Dutch inundations differs from ordinary swampy and impassable wet low lands in the following respects:—
1. The soil itself is dry and consists either of dry meadows or of cultivated fields.
2. For purposes of irrigation or of drainage, a number of small ditches of greater or less depth and breadth intersect the country in such a way that they may be seen running in lines in parallel directions.
3. Larger canals, enclosed by dykes and intended for irrigation, drainage, and transit of vessels, run through the country in all possible directions and are of such a size that they can only be passed on bridges.
4. The level of the ground throughout the whole district subject to inundation, lies perceptibly under the level of the sea, therefore, of course, under that of the canals.
5. The consequence of this is, that by means of cutting the dams, closing and opening the sluices, the whole country can be laid under water, so that there are no dry roads except on the tops of the dykes, all others being either entirely under water or, at least, so soaked that they become no longer fit for use. Now, if even the inundation is only three or four feet deep, so that, perhaps, for short distances it might be waded through, still even that is made impossible on account of the smaller ditches mentioned under No. 2, which are not visible. It is only where these ditches have a corresponding direction, so that we can move between two of them without crossing either, that the inundation does not constitute in effect an absolute bar to all communication. It is easy to conceive that this exception to the general obstruction can only be for short distances, and, therefore, can only be used for tactical purposes of an entirely special character.
From all this we deduce—
1. That the assailant’s means of moving are limited to a more or less small number of practicable lines, which run along very narrow dykes, and usually have a wet ditch on the right and left, consequently form very long defiles.
2. That every defensive preparation upon such a dam may be easily strengthened to such a degree as to become impregnable.
3. But that, because the defensive is so hemmed in, he must confine himself to the most passive resistance as respects each isolated point, and consequently must look for his safety entirely from passive resistance.
4. That in such a country it is not a system of a single defensive line, closing the country like a simple barrier, but that as in every direction the same obstacle to movement exists, and the same security for flanks may be found, new posts may incessantly be formed, and in this manner any portion of the first defensive line, if lost, may be replaced by a new piece. We may say that the number of combinations here, like those on a chessboard, are infinite.
5. But while this general condition of a country is only conceivable along with the supposition of a high degree of cultivation and a dense population, it follows of itself that the number of passages, and therefore the number of posts required for their defence, must be very great in comparison to other strategetic dispositions; from which again we have, as a consequence, that such a defensive line must not be long.
The principal line of defence in Holland is from Naarden on the Zuyder Zee (the greater part of the way behind the Vecht), to Gorcum on the Waal, that is properly to the Biesbosch, its extent being about forty miles. For the defence of this line a force of 25,000 to 30,000 was employed in 1672, and again in 1787. If we could reckon with certainty upon an invincible resistance, the results would certainly be very great, at least for the provinces of Holland lying behind that line.
In 1672 the line actually withstood very superior forces led by great Generals, first Condé, and afterwards Luxembourg, who had under their command 40,000 to 50,000 men, and yet would not assault, preferring to wait for the winter which did not prove severe enough. On the other hand, the resistance which was made on this first line in 1787 amounted to nothing, and even that which was made by a second line much shorter, between the Zuyder Zee and the lake of Haarlem, although somewhat more effective, was overcome by the Duke of Brunswick in one day, through a very skilful tactical disposition well adapted to the locality, and this although the Prussian force actually engaged in the attack was little, if at all, superior in numbers to the troops guarding the lines.
The different result in the two cases is to be attributed to the difference in the supreme command. In the year 1672 the Dutch were surprised by Louis XIV., while everything was on a peace establishment, in which, as is well known, there breathed very little military spirit as far as concerned land forces. For that reason the greater number of the fortresses were deficient in all articles of material and equipment, garrisoned only by weak bodies of hired troops, and defended by governors who were either native-born incapables, or treacherous foreigners. Thus all the Brandenburg fortresses on the Rhine, garrisoned by Dutch, as well as all their own places situated to the east of the line of defence above described, except Groningen, very soon fell into the hands of the French, and for the most part without any real defence. And in the conquest of this great number of places consisted the chief exertions of the French army, 150,000 strong, at that time.
But when, after the murder of the brothers De Witt, in August 1672, the Prince of Orange came to the head of affairs, bringing unity to the measures for national defence, there was still time to close the defensive line above mentioned, and all the measures then adopted harmonised so well with each other that neither Condé nor Luxembourg, who commanded the French forces left in Holland after the departure of the two Armies under Turenne and Louis in person, would venture to attempt anything against the separate posts.
In the year 1787 all was different. It was not the Republic of seven united provinces, but only the province of Holland which had to resist the invasion. The conquest of all the fortresses, which had been the principal object in 1672, was therefore not the question; the defence was confined at once to the line we have described. But the assailant this time, instead of 150,000 men, had only 25,000 and was no mighty sovereign of a great country adjoining Holland, but the subordinate General of a distant Prince, himself by no means independent in many respects. The people in Holland, like those everywhere else at that time, were divided into two parties, but the republican spirit in Holland was decidedly predominant, and had at the same time attained even to a kind of enthusiastic excitement. Under these circumstances the resistance in the year 1787 ought to have ensured at least as great results as that of 1672. But there was one important difference, which is, that in the year 1787 unity of command was entirely wanting. What in 1672 had been left to the wise, skilful, and energetic guidance of the Prince of Orange, was entrusted to a so-called Defence Commission in 1787, which although it included in its number men of energy, was not in a position to infuse into its work the requisite unity of measures, and to inspire others with that confidence which was wanted to prevent the whole instrument from proving imperfect and inefficient in use.
We have dwelt for a moment on this example, in order to give more distinctness to the conception of this defensive measure, and at the same time to show the difference in the effects produced, according as more or less unity and sequence prevail in the direction of the whole.
Although the organisation and method of defence of such a defensive line are tactical subjects, still, in connection with the latter, which is the nearest allied to Strategy, we cannot omit to make an observation to which the campaign of 1787 gives occasion.
We think, namely, that however passive the defence must naturally be at each point in a line of this kind, still an offensive action from some one point of the line is not impossible, and may not be unproductive of good results if the enemy, as was the case in 1787, is not decidedly very superior. For although such an attack must be executed by means of dykes, and on that account cannot certainly have the advantage of much freedom of movement or of any great impulsive force, nevertheless, it is impossible for the offensive side to occupy all the dykes and roads which he does not require for his own purposes, and therefore the defensive with his better knowledge of the country, and being in possession of the strong points, should be able by some of the unoccupied dykes to effect a real flank attack against the columns of the assailant, or to cut them off from their sources of supply. If now, on the other hand, we reflect for a moment on the constrained position in which the assailant is placed, how much more dependent he is on his communications than in almost any other conceivable case, we may well imagine that every sally on the part of the defensive side which has the remotest possibility of success must at once as a demonstration be most effective. We doubt very much if the prudent and cautious Duke of Brunswick would have ventured to approach Amsterdam if the Dutch had only made such a demonstration, from Utrecht for instance.