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 corps 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 corps by its position has less power of resistance in itself, and is less easily reinforced, its retreat must be made more rapidly in proportion than if it stood nearer the main body, and had 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 advanced corps 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 two 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.
(*) Here, as well as elsewhere, by the word mile, the German mile is meant.—Tr.
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 three or four 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 a mile 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 four miles, for which usually ten hours are required, then from the moment that the enemy appears in force in front of the advanced corps, we may reckon upon fifteen hours before he is in a condition to attack our main body. On the other hand, if the advanced guard is posted only a mile 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 advanced 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 advanced 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 advanced guard thrown further forward.