There are only two ways of insuring the success of a siege. The first, to begin by beating the enemy’s army employed to cover the place, forcing it out of the field, and throwing its remains beyond some great natural obstacle, such as a chain of mountains, or large river. Having accomplished this object, an army of observation should be placed behind the natural obstacle, until the trenches are finished and the place taken.
But if it be desired to take the place in presence of a relieving army, without risking a battle, then the whole materiel and equipment for a siege are necessary to begin with, together with ammunition and provisions for the presumed period of its duration, and also lines of contravallation and circumvallation, aided by all the localities of heights, woods, marshes and inundations.
Having no longer occasion to keep up communications with your depôts, it is now only requisite to hold in check the relieving army. For this purpose, an army of observation should be formed, whose business it is never to lose sight of that of the enemy, and which, while it effectually bars all access to the place, has always time enough to arrive upon his flanks or rear in case he should attempt to steal a march.
It is to be remembered, too, that by profiting judiciously by the lines of contravallation, a portion of the besieging army will always be available in giving battle to the approaching enemy.
Upon the same general principle, when a place is to be besieged in presence of an enemy’s army, it is necessary to cover the siege by lines of circumvallation.