attack like blocks of ice. The enemy must at least invest them, and requires for that, if the garrisons are brave and enterprising, perhaps double their strength. But, besides, these garrisons may and do mostly consist in part of troops, who, although competent to duty in a garrison, are not fit for the field—half trained militia, invalids, convalescents, armed citizens, landsturm, etc. The enemy, therefore, in such case is perhaps weakened four times more than we are.
This disproportionate weakening of the enemy’s power is the first and most important but not the only advantage which a besieged fortress affords by its resistance. From the moment that the enemy crosses our line of fortresses, all his movements become much more constrained; he is limited in his lines of retreat, and must constantly attend to the direct covering of the sieges which he undertakes.
Here, therefore, fortresses co-operate with the defensive act in a most extensive and decisive manner, and of all the objects that they can have, this may be regarded as the most important.
If this use of fortresses—far from being seen regularly repeating itself—seldom comparatively occurs in military history, the cause is to be found in the character of most wars, this means being to a certain extent far too decisive and too thoroughly effectual for them, the explanation of which we leave till hereafter.
In this use of fortresses it is chiefly their offensive power that is called for, at least it is that by which their effectual action is chiefly produced. If a fortress was no more to an aggressor than a point which could not be occupied by him, it might be an obstacle to him, but not to such a degree as to compel him to lay siege to it But as he cannot leave six, eight, or ten thousand men to do as they like in his rear, he is obliged to invest the place with a sufficient force, and if he desires that this investment should