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_.
If the besieging force is of numerical strength enough (after leaving
a corps before the place four times the amount of the garrison) to
cope with the relieving army, it may remove more than one day’s march
from the place; but if it be inferior in numbers after providing for
the siege, as above stated, it should remain only a short day’s march
from the spot, in order to fall back upon its lines, if necessary, or
receive succor in case of attack.
If the investing corps and army of observation are only equal when
united to the relieving force, the besieging army should remain entire
within, or near its lines, and push the works and the siege with the
greatest activity.