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.