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, and the most extended and compulsory positions often become necessary in order to cover depots or sources of supply.
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; as such their possession and loss affects the army directly. Besides this, it is also always worth while to preserve this portion of the national wealth, partly on account of the resources which they furnish directly, partly because, in negotiations for peace, an important place is in itself a valuable weight thrown into the scale.
This use of fortresses has been too little regarded in modern times, and yet it is one of the most natural, and one which has a most powerful effect, and is the least liable to mistakes. If there was a country in which not only all great and rich cities, but all populous places as well were fortified, and defended by the inhabitants and the people belonging to the adjacent districts, then by that means the expedition of military operation would be so much reduced, and the people attacked would press with so great a part of their whole weight in the scales, that the talent as well as the force of will of the enemy’s general would sink to nothing.
We just mention this ideal application of fortification to a country to do justice to what we have just supposed to be the proper use of fortresses, and that the importance of the direct protection which they afford may not be overlooked for a moment; but in any other respect this idea will not again interrupt our considerations, for amongst the whole number of fortresses there must always be some which must be more strongly fortified than others, to serve as the real supports of the active army.
The purposes specified under 1 and 2 hardly call forth any other but the passive action of fortresses.
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.
If the country is in the least degree difficult, there are often delays connected with the slightest deviation of the road which may cause the loss of a whole day’s march, and, if the road is much used, may become of great importance.
How they may have an influence on enterprises by closing the navigation of a river is clear in itself.