Front Page Titles (by Subject) CHAPTER VIII: MODE OF ACTION OF ADVANCED CORPS - On War, vol. 2
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CHAPTER VIII: MODE OF ACTION OF ADVANCED CORPS - 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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MODE OF ACTION OF ADVANCED CORPS
We have just seen how the security of the Army is expected, from the effect which an advance guard and flank corps produce on an advancing enemy. Such bodies are always to be considered as very weak whenever we imagine them in conflict with the main body of the enemy, and therefore a peculiar mode of using them is required, that they may fulfil the purpose for which they are intended, without incurring the risk of the serious loss which is to be feared from this disproportion in strength.
The object of a detachment of this description, is to observe the enemy, and to delay his progress.
For the first of these purposes a smaller body would never be sufficient, partly because it would be more easily driven back, partly because its means of observation—that is its eyes—could not reach as far.
But the observation must be carried to a high point; the enemy must be made to develop his whole strength before such a Corps, and thereby reveal to a certain extent, not only his force, but also his plans.
For this its mere presence would be sufficient, and it would only be necessary to wait and see the measures by which the enemy seeks to drive it back, and then commence its retreat at once.
But further, it must also delay the advance of the enemy, and that implies actual resistance.
Now how can we conceive this waiting until the last moment, as well as this resistance, without such a body being in constant danger of serious loss? Chiefly in this way, that the enemy himself is preceded by an advance guard, and therefore does not advance at once with all the outflanking and overpowering weight of his whole force. Now, if this advance guard is also from the commencement superior to our advanced corps, as we may naturally suppose it is intended it should be, and if the enemy’s main body is also nearer to his advance guard than we are to ours, and if that main body, being already on the march, will soon be on the spot to support the attack of his advance guard with all his strength; still this first act, in which our advanced corps has to contend with the enemy’s advance guard, that is with a force not much exceeding its own, ensures at once a certain gain of time, and thus allows of our watching the adversary’s movements for some time without endangering our own retreat.
But even a certain amount of resistance which such a force can offer in a suitable position is not attended with such disadvantage as we might anticipate in other cases through the disproportion in the strength of the forces engaged. The chief danger in a contest with a superior enemy consists always in the possibility of being turned and placed in a critical situation by the enemy enveloping our position; but in the case to which our attention is now directed, a risk of this description is very much less, owing to the advancing enemy never knowing exactly how near at hand support from the main body of his opponent’s Army itself may be, which may place his advanced column between two fires. The consequence is, that the enemy in advancing keeps the heads of his single columns as nearly as possible in line, and only begins very cautiously to attempt to turn one or other wing after he has sufficiently reconnoitred our position. While the enemy is thus feeling about and moving guardedly, the Corps we have thrown forward has time to fall back before it is in any serious danger.
As for the length of the resistance which such a Corps should offer against the attack in front, or against the commencement of any turning movement, that depends chiefly on the nature of the ground and the proximity of the enemy’s supports. If this resistance is continued beyond its natural measure, either from want of judgment or from a sacrifice being necessary in order to give the main body the time it requires, the consequence must always be a very considerable loss.
It is only in rare instances, and more especially when some local obstacle is favourable, that the resistance actually made in such a combat can be of importance, and the duration of the little battle of such a Corps would in itself be hardly sufficient to gain the time required; that time is really gained in a threefold manner, which lies in the nature of the thing, viz.:
1. By the more cautious, and consequently slower advance of the enemy.
2. By the duration of the actual resistance offered.
3. By the retreat itself.
This retreat must be made as slowly as is consistent with safety. If the country affords good positions they should be made use of, as that obliges the enemy to organise fresh attacks and plans for turning movements, and by that means more time is gained. Perhaps in a new position a real combat even may again be fought.
We see that the opposition to the enemy’s progress by actual fighting and the retreat are completely combined with one another, and that the shortness of the duration of the fights must be made up for by their frequent repetition.
This is the kind of resistance which an advanced force should offer. The degree of effect depends chiefly on the strength of the Corps, and the configuration of the country; next on the length of the road which the Corps has to march over, and the support which it receives.
A small body, even when the forces on both sides are equal, can never make as long a stand as a considerable Corps; for the larger the masses the more time they require to complete their action, of whatever kind it may be. In a mountainous country the mere marching is of itself slower, the resistance in the different positions longer, and attended with less danger, and at every step favourable positions may be found.
As the distance to which a detachment is pushed forward increases so will the length of its retreat, and therefore also the absolute gain of time by its resistance; but as such a body by its position has less power of resistance in itself, and is less easily reinforced, its retreat must be made more rapidly in proportion as it is nearer the main body, and has a shorter distance to traverse.
The support and means of rallying afforded to an advanced Corps must naturally have an influence on the duration of the resistance, as all the time that prudence requires for the security of the retreat is so much taken from the resistance, and therefore diminishes its amount.
There is a marked difference in the time gained by the resistance of an advance guard when the enemy makes his first appearance after midday; in such a case the length of the night is so much additional time gained, as the advance is seldom continued throughout the night. Thus it was that, in 1815, on the short distance from Charleroi to Ligny, not more than ten miles, the first Prussian Corps under General Ziethen, about 30,000 strong, against Buonaparte at the head of 120,000 men, was enabled to gain twenty-four hours for the Prussian Army then engaged in concentrating. The first attack was made on General Ziethen about nine o’clock on the morning of 15th June, and the battle of Ligny did not commence until about two on the afternoon of 16th. General Ziethen suffered, it is true, very considerable loss, amounting to five or six thousand men killed, wounded, or prisoners.
If we refer to experience the following are the results, which may serve as a basis in any calculations of this kind.
A Division of ten or twelve thousand men, with a proportion of cavalry, a day’s march of fifteen to twenty miles in advance in an ordinary country, not particularly strong, will be able to detain the enemy (including time occupied in the retreat) about half as long again as he would otherwise require to march over the same ground, but if the Division is only five miles in advance, then the enemy ought to be detained about twice or three times as long as he otherwise would be on the march.
Therefore supposing the distance to be a march of twenty miles, for which usually ten hours are required, then from the moment that the enemy appears in force in front of the advanced body, we may reckon upon fifteen hours before he is in a condition to attack our main Army. On the other hand, if the advance guard is posted only five miles in advance, then the time which will elapse before our Army can be attacked will be more than three or four hours, and may very easily come up to double that, for the enemy still requires just as much time to mature his first measures against our advance guard, and the resistance offered by that guard in its original position will be greater than it would be in a position further forward.
The consequence is, that in the first of these supposed cases the enemy cannot easily make an attack on our main body on the same day that he presses back the advanced Corps, and this exactly coincides with the results of experience. Even in the second case the enemy must succeed in driving our advance guard from its ground in the first half of the day to have the requisite time for a general action.
As the night comes to our help in the first of these supposed cases, we see how much time may be gained by an advance guard thrown further forward.
With reference to troops placed on the sides or flanks, the object of which we have before explained, the mode of action is in most cases more or less connected with circumstances which belong to the province of immediate application. The simplest way is to look upon them as advance guards placed on the sides, which being at the same time thrown out somewhat in advance, retreat in an oblique direction upon the Army.
As these bodies are not immediately in the front of the Army, and cannot be so easily supported as a regular advance guard, they would, therefore, be exposed to greater danger if it was not that the enemy’s offensive power in most cases is somewhat less at the outer extremities of his line, and in the worst cases such detachments have sufficient room to give way without exposing the Army so directly to danger as a flying advance guard might do by its rapid retreat.
The most usual and best means of supporting an advanced Corps is by a considerable body of cavalry, for which reason, when necessary from the distance at which the Corps is advanced, the reserve cavalry is posted between the main body and the advanced Corps.
The conclusion to be drawn from the preceding reflections is, that an advanced Corps effects more by its presence than by its efforts, less by the combats in which it engages than by the possibility of those in which it might engage: that it should never attempt to stop the enemy’s movements, but only serve like a pendulum to moderate and regulate them, so that they may be made matter of calculation.