THE attack of fortresses cannot of course come before us here in its aspect as a branch of the science of fortification or military works; we have only to consider the subject, first, in its relation to the strategic object with which it is connected; secondly, as regards the choice among several fortresses; and thirdly, as regards the manner in which a siege should be covered.
That the loss of a fortress weakens the defence, especially in case it forms an essential part of that defence; that many conveniences accrue to the assailant by gaining possession of one, inasmuch as he can use it for magazines and depôts, and by means of it can cover districts of country cantonments, etc.; that if his offensive at last should have to be changed into the defensive, it forms the very best support for that defensive—all these relations which fortresses bear to theatres of war, in the course of a war, make themselves sufficiently evident by what has been said about fortresses in the book on the Defence, the reflection from which throws all the light required on these relations with the attack.
In relation to the taking of strong places, there is also a great difference between campaigns which tend to a great decision and others. In the first, a conquest of this description is always to be regarded as an evil which is unavoidable. As long as there is yet a decision to be made, we undertake no sieges but such as are positively unavoidable. When the decision has been already given—the crisis, the utmost tension of forces, some time passed—and when, therefore, a state of rest has commenced, then the capture of strong places serves as a consolidation of the conquests made, and then they can generally be carried out, if not without effort and expenditure of force, at least without danger. In the crisis itself the siege of a fortress heightens the intensity of the crisis to the prejudice of the offensive; it is evident that nothing so much weakens the force of the offensive, and therefore there is nothing so certain to rob it of its preponderance for a season. But there are cases in which the capture of this or that fortress is quite unavoidable, if the offensive is to be continued, and in such case a siege is to be considered as an intensified progress of the attack; the crisis will be so much greater the less there has been decided previously. All that remains now for consideration on this subject belongs to the book on the plan of the war.
In campaigns with a limited object, a fortress is generally not the means but the end itself; it is regarded as a small independent conquest, and as such has the following advantages over every other:—
1. That a fortress is a small, distinctly-defined conquest, which does not require a further expenditure of force, and therefore gives no cause to fear a reaction.
2. That in negotiating for peace, its value as an equivalent may be turned to account.
3. That a siege is a real progress of the attack, or at least seems so, without constantly diminishing the force like every other advance of the offensive.
4. That the siege is an enterprise without a catastrophe.
The result of these things is that the capture of one or more of the enemy's strong places, is very frequently the object of those strategic attacks which cannot aim at any higher object.
The grounds which decide the choice of the fortress which should be attacked, in case that may be doubtful, generally are—
(a) That it is one which can be easily kept, therefore stands high in value as an equivalent in case of negotiations for peace.
(b) That the means of taking it are at hand. Small means are only sufficient to take small places; but it is better to take a small one than to fail before a large one.
(c) Its strength in engineering respects, which obviously is not always in proportion to its importance in other respects. Nothing is more absurd than to waste forces before a very strong place of little importance, if a place of less strength may be made the object of attack.
(d) The strength of the armament and of the garrison as well. If a fortress is weakly armed and insufficiently garrisoned, its capture must naturally be easier; but here we must observe that the strength of the garrison and armament, are to be reckoned amongst those things which make up the total importance of the place, because garrison and armaments are directly parts of the enemy's military strength, which cannot be said in the same measure of works of fortification. The conquest of a fortress with a strong garrison can, therefore, much more readily repay the sacrifice it costs than one with very strong works.
(e) The facility of moving the siege train. Most sieges fail for want of means, and the means are generally wanting from the difficulty attending their transport. Eugene's siege of Landreci, 1712, and Frederick the Great's siege of Olmutz, 1758, are very remarkable instances in point.
(f) Lastly, there remains the facility of covering the siege as a point now to be considered.
There are two essentially different ways by which a siege may be covered: by entrenching the besieging force, that is, by a line of circumvallation, and by what is called lines of observation. The first of these methods has gone quite out of fashion, although evidently one important point speaks in its favour, namely, that by this method the force of the assailant does not suffer by division exactly that weakening which is so generally found a great disadvantage at sieges. But we grant there is still a weakening in another way, to a very considerable degree, because—
1. The position round the fortress, as a rule, is of too great extent for the strength of the army.
2. The garrison, the strength of which, added to that of the relieving army, would only make up the force originally opposed to us, under these circumstances is to be looked upon as an enemy's corps in the middle of our camp, which, protected by its walls, is invulnerable, or at least not to be overpowered, by which its power is immensely increased.
3. The defence of a line of circumvallation admits of nothing but the most absolute defensive, because the circular order, facing outwards, is the weakest and most disadvantageous of all possible orders of battle, and is particularly unfavourable to any advantageous counter-attacks. There is no alternative, in fact, but to defend ourselves to the last extremity within the entrenchments. That these circumstances may cause a greater diminution of the army than one-third which, perhaps, would be occasioned by forming an army of observation, is easy to conceive. If, added to that, we now think of the general preference which has existed since the time of Frederick the Great for the offensive, as it is called, (but which, in reality, is not always so) for movements and manœuvres, and the aversion to entrenchments, we shall not wonder at lines of circumvallation having gone quite out of fashion. But this weakening of the tactical resistance is by no means its only disadvantage; and we have only reckoned up the prejudices which forced themselves into the judgment on the lines of circumvallation next in order after that disadvantage, because they are nearly akin to each other. A line of circumvallation only in reality covers that portion of the theatre of war which it actually encloses; all the rest is more or less given up to the enemy if special detachments are not made use of to cover it, in which way the very partition of force which it was intended to obviate takes place. Thus the besieging army will be always in anxiety and embarrassment on account of the convoys which it requires, and the covering the same by lines of circumvallation, is not to be thought of if the army and the siege supplies required are considerable, and the enemy is in the field in strong force, unless under such conditions as are found in the Netherlands, where there is a whole system of fortresses lying close to each other, and intermediate lines connecting them, which cover the rest of the theatre of war, and considerably shorten the lines by which transport can be affected. In the time of Louis the Fourteenth the conception of a theatre of war had not yet bound itself up with the position of an army. In the Thirty Years' War particularly, the armies moved here and there sporadically before this or that fortress, in the neighbourhood of which there was no enemy's corps at all, and besieged it as long as the siege equipment they had brought with them lasted, and until an enemy's army approached to relieve the place. Then lines of circumvallation had their foundation in the nature of circumstances.
In future it is not likely they will be often used again, unless where the enemy in the field is very weak, or the conception of the theatre of war vanishes before that of the siege. Then it will be natural to keep all the forces united in the siege, as a siege by that means unquestionably gains in energy in a high degree.
The lines of circumvallation in the reign of Louis XIV., at Cambray and Valenciennes, were of little use, as the former were stormed by Turenne, opposed to Condé, the latter by Condé opposed to Turenne; but we must not overlook the endless number of other cases in which they were respected, even when there existed in the place the most urgent need for relief; and when the commander on the defensive side was a man of great enterprise, as in 1708, when Villars did not venture to attack the allies in their lines at Lille. Frederick the Great at Olmutz, 1758, and at Dresden, 1760, although he had no regular lines of circumvallation, had a system which in all essentials was identical; he used the same army to carry on the siege, and also as a covering army. The distance of the Austrian army induced him to adopt this plan at Olmutz, but the loss of his convoy at Domstadtel made him repent it; at Dresden in 1760 the motives which led him to this mode of proceeding, were his contempt for the German States' imperial army, and his desire to take Dresden as soon as possible.
Lastly, it is a disadvantage in lines of circumvallation, that in case of a reverse it is more difficult to save the siege train. If a defeat is sustained at a distance of one or more days' march from the place besieged, the siege may be raised before the enemy can arrive, and the heavy trains may, in the mean time, gain also a day's march.
In taking up a position for an army of observation, an important question to be considered is the distance at which it should be placed from the besieged place. This question will, in most cases, be decided by the nature of the country, or by the position of other armies or corps with which the besiegers have to remain in communication. In other respects, it is easy to see that, with a greater distance, the siege is better covered, but that by a smaller distance, not exceeding a few miles, the two armies are better able to afford each other mutual support.