If this use of fortresses—far from being seen regularly repeating itself—seldom comparatively occurs in military history, the cause is to be found in the character of most wars, this means being to a certain extent far too decisive and too thoroughly effectual for them, the explanation of which we leave till hereafter.
In this use of fortresses it is chiefly their offensive power that is called for, at least it is that by which their effectual action is chiefly produced. If a fortress was no more to an aggressor than a point which could not be occupied by him, it might be an obstacle to him, but not to such a degree as to compel him to lay siege to it But as he cannot leave six, eight, or ten thousand men to do as they like in his rear, he is obliged to invest the place with a sufficient force, and if he desires that this investment should not continue to employ so large a detachment, he must convert the investment into a siege, and take the place. From the moment the siege commences, it is then chiefly the passive efficacy of the fortress which comes into action.
All the destinations of fortresses which we have been hitherto considering are fulfilled in a simple and mainly in a direct manner. On the other hand, in the next two objects the method of action is more complicated.
8. As a protection to extended cantonments. That a moderate-sized fortress closes the approach to cantonments lying behind it for a width of three or four milesis a simple result of its existence; but how such a place comes to have the honour of covering a line of cantonments fifteen or twenty miles in length, which we find frequently spoken of in military history as a fact—that requires investigation as far as it has really taken place, and refutation so far as it may be mere illusion.
The following points offer themselves for consideration:—
(1.) That the place in itself blocks one of the main roads, and really covers a breadth of three or four miles of country.
(2.) That it may be regarded as an exceptionally strong advanced post, or that it affords a more complete observation of the country, to which may be added facilities in the way of secret information through the ordinary relations of civil life which exist between a great town and the adjacent districts It is natural that in a place of six, eight or ten thousand inhabitants, one should be able to learn more of what is going on in the neighbourhood than in a mere village, the quarters of an ordinary outpost.
(3.) That smaller corps are appuyed on it, derive from it protection and security, and from time to time can advance towards the enemy, it may be to bring in intelligence, or, in case he attempts to turn the fortress, to underdertake something against his rear; that therefore although a fortress, cannot quit its place, still it may have the efficacy of an advanced corps (Fifth Book, eighth Chapter).
(4.) That the defender, after assembling his corps, can take up his position at a point directly behind this fortress, which the assailant cannot reach without becoming exposed to danger from the fortress in his rear.
No doubt every attack on a line of cantonments as such is to be taken in the sense of a surprise, or rather, we are only speaking here of that kind of attack; now it is evident in itself that an attack by surprise accomplishes its effect in a much shorter space of time than a regular attack on a theatre of war. Therefore, although in the latter case, a fortress which is to be passed by must necessarily be invested and kept in check, this investment will not be so indispensable in the case of a mere sudden attack on cantonments, and therefore in the same proportion the fortress will be less an obstacle to the attack of the cantonments. That is true enough; also the cantonments lying at a distance of six to eight miles from the fortress cannot be directly protected by it; but the object of such a sudden attack does not consist alone in the attack of a few cantonments. Until we reach the book on attack we cannot describe circumstantially the real object of such a sudden attack and what may be expected from it; but this much we may say at present, that its principal results are obtained, not by the actual attack on some isolated quarters, but by the series of combats which the aggressor forces on single corps not in proper order, and more bent upon hurrying to certain points than upon fighting. But this attack and pursuit will always be in a direction more or less towards the centre of the enemy’s cantonments, and, therefore, an important fortress lying before this centre will certainly prove a very great impediment to the attack.