But even the smallest active element of the weakest garrison, is still essential for the different objects which fortresses are destined to fulfil, for strictly speaking even the most passive of all the functions of a fortress (defence against attack) cannot be imagined exclusive of that active agency. 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.
- 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, and the most extended and compulsory positions often become necessary in order to cover depots or sources of supply.