An army on the defensive without fortresses has a hundred vulnerable spots; it is a body without armour.
- 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.