At first sight it appears somewhat preposterous to protect a fortress from the enemy’s attack by placing an army in front of it; such a measure looks like a kind of pleonasm, as fortifications are built to resist a hostile attack of themselves. Yet it is a measure which we see resorted to thousands and thousands of times. But thus it is in the conduct of war; the most common things often seem the most incomprehensible. Who would presume to pronounce these thousands of instances to be so many blunders on the ground of this seeming inconsistency? The constant repetition of the measure shows that it must proceed from some deep-seated motive. This reason is, however, no other than that pointed out above, emanating from moral sluggishness and inactivity.
If the defender places himself in front of his fortress, the enemy cannot attack it unless he first beats the army in front of it; but a battle is a decision; if that is not the enemy’s object then there will be no battle, and the defender will remain in possession of his fortress without striking a blow; consequently, whenever we do not believe the enemy intends to fight a battle, we should venture on the chance of his not making up his mind to do so, especially as in most cases we still retain the power of withdrawing behind the fortress in a moment, if, contrary to our expectation, the enemy should march to attack us; the position before the fortress is in this way free from danger, and the probability of maintaining the status quo without any sacrifice, is not even attended with the slightest risk.
If the defender places himself behind the fortress, he offers the assailant an object which is exactly suited to the circumstances in which the latter is placed. If the fortress is not of great strength, and he is not quite unprepared, he will commence the siege: in order that this may not end in the fall of the place, the defender must march to its relief. The positive action, the initiative, is now laid on him, and the adversary who by his siege is to be regarded as advancing towards his object, is in the situation of occupier.