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This active element consists in the attacks which the garrison may undertake upon every enemy who
approaches within a certain distance. The larger the garrison, so much the stronger numerically will be
the detachments that may be employed on such expeditions, and the stronger such detachments the
wider as a rule will be the range of their operations.
At the same time it is evident that amongst the different purposes which a fortress may have to answer generally, or in this or that moment, the passive element will be most required at one time, the active at another. The role which a fortress is to fulfil may be perfectly simple, and the action of the place will in such case be to a certain extent direct; it may be partly complicated, and the action then becomes more or less indirect. We shall examine these subjects separately, commencing with the first; but at the outset we must state that a fortress may be intended to answer several of these purposes, perhaps all of them, either at once, or at least at different stages of the war.
We say, therefore, that fortresses are great and most important supports of the defensive.
1 As secure depots of stores of all kinds. The assailant during his aggression subsists his army from day to day; the defensive usually must have made preparations long beforehand, he need not therefore draw provisions exclusively from the district he occupies, and which he no doubt desires to spare. Storehouses are therefore for him a great necessity. The provisions of all kinds which the aggressor possesses are in his rear as he advances, and are therefore exempt from the dangers of the theatre of war, while those of the defensive are exposed to them. If these provisions of all kinds are not in fortified places, then a most injurious effect on the operations in the field is the consequence ...
An army on the defensive without fortresses has a hundred vulnerable spots; it is a body without armour.
2 As a protection to great and wealthy towns. This purpose is closely allied to the first, for great and
wealthy towns, especially commercial ones, are the natural storehouses of an army.
3 As real barriers, they close the roads, and in most cases the rivers, on which they are situated.
It is not as easy as is generally supposed to find a practicable lateral road which passes round a fortress,
for this turning must be made, not only out of reach of the guns of this place, but also by a detour greater
or less, to avoid sorties of the garrison.
4 As tactical points dappui. As the diameter of the zone covered by the fire of even a very inferior
class of fortifications is usually some miles, fortresses may be considered always as the best points
dappui for the flanks of a position.
5 As a station (or stage). If fortresses are on the line of communication of the defensive, as is generally the case, they serve as halting places for all that passes up and down these lines ... If a valuable convoy ... can reach a fortress by hastening the march or quickly turning, it is saved, and may wait there till the danger is past. Further, all troops marching to or from the army, after halting here for a few days, are better able to hasten the remainder of the march, and a halting day is just the time of greatest danger. In this way a fortress situated half way on a line of communication of one hundred and fifty miles shortens the line in a manner one half.
6 As places of refuge for weak or defeated corps. Under the guns of a moderate sized fortress every
corps is safe from the enemys blows, even if no entrenched camp is specially prepared for them.
7 As a real shield against the enemys aggression. Fortresses which the defender leaves in his front break the stream of the enemys attack like ice breakers on the piers of a bridge. The enemy must at