That the loss of a fortress weakens the defence, especially in case it forms an essential part of that defence; that many conveniences accrue to the assailant by gaining possession of one, inasmuch as he can use it for magazines and depôts, and by means of it can cover districts of country cantonments, etc.; that if his offensive at last should have to be changed into the defensive, it forms the very best support for that defensive—all these relations which fortresses bear to theatres of war, in the course of a war, make themselves sufficiently evident by what has been said about fortresses in the book on the Defence, the reflection from which throws all the light required on these relations with the attack.
In relation to the taking of strong places, there is also a great difference between campaigns which tend to a great decision and others. In the first, a conquest of this description is always to be regarded as an evil which is unavoidable. As long as there is yet a decision to be made, we undertake no sieges but such as are positively unavoidable. When the decision has been already given—the crisis, the utmost tension of forces, some time passed—and when, therefore, a state of rest has commenced, then the capture of strong places serves as a consolidation of the conquests made, and then they can generally be carried out, if not without effort and expenditure of force, at least without danger. In the crisis itself the siege of a fortress heightens the intensity of the crisis to the prejudice of the offensive; it is evident that nothing so much weakens the force of the offensive, and therefore there is nothing so certain to rob it of its preponderance for a season. But there are cases in which the capture of this or that fortress is quite unavoidable, if the offensive is to be continued, and in such case a siege is to be considered as an intensified progress of the attack; the crisis will be so much greater the less there has been decided previously. All that remains now for consideration on this subject belongs to the book on the plan of the war.
In campaigns with a limited object, a fortress is generally not the means but the end itself; it is regarded as a small independent conquest, and as such has the following advantages over every other:—
1. That a fortress is a small, distinctly-defined conquest, which does not require a further expenditure of force, and therefore gives no cause to fear a reaction.
2. That in negotiating for peace, its value as an equivalent may be turned to account.
3. That a siege is a real progress of the attack, or at least seems so, without constantly diminishing the force like every other advance of the offensive.
4. That the siege is an enterprise without a catastrophe.
The result of these things is that the capture of one or more of the enemy’s strong places, is very frequently the object of those strategic attacks which cannot aim at any higher object.
The grounds which decide the choice of the fortress which should be attacked, in case that may be doubtful, generally are—
(a) That it is one which can be easily kept, therefore stands high in value as an equivalent in case of negotiations for peace.