The attack on fortresses cannot of course come before us here in its
aspect as a branch of the science of fortification or military works;
we have only to consider the subject, first, in its relation to the
strategic object with which it is connected; secondly, as regards the
choice among several fortresses; and thirdly, as regards the manner in
which a siege should be covered.
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.
(_b_) That the means of taking it are at hand. Small means are only
sufficient to take small places; but it is better to take a small one
than to fail before a large one.
(_c_) Its strength in engineering respects, which obviously is not
always in proportion to its importance in other respects. Nothing is
more absurd than to waste forces before a very strong place of little
importance, if a place of less strength may be made the object of
attack.
(_d_) The strength of the armament and of the garrison as well. If a
fortress is weakly armed and insufficiently garrisoned, its capture
must naturally be easier; but here we must observe that the strength of
the garrison and armament, are to be reckoned amongst those things
which make up the total importance of the place, because garrison and
armaments are directly parts of the enemy’s military strength, which
cannot be said in the same measure of works of fortification. The
conquest of a fortress with a strong garrison can, therefore, much more
readily repay the sacrifice it costs than one with very strong works.
(_e_) The facility of moving the siege train. Most sieges fail for want
of means, and the means are generally wanting from the difficulty
attending their transport. Eugene’s siege of Landreci, 1712, and
Frederick the Great’s siege of Olmütz, 1758, are very remarkable
instances in point.
(_f_) Lastly, there remains the facility of covering the siege as a
point now to be considered.
There are two essentially different ways by which a siege may be
covered: by entrenching the besieging force, that is, by a line of
circumvallation, and by what is called lines of observation. The first
of these methods has gone quite out of fashion, although evidently one
important point speaks in its favour, namely, that by this method the
force of the assailant does not suffer by division exactly that
weakening which is so generally found a great disadvantage at sieges.
But we grant there is still a weakening in another way, to a very
considerable degree, because—
1. The position round the fortress, as a rule, is of too great extent
for the strength of the army.
2. The garrison, the strength of which, added to that of the relieving
army, would only make up the force originally opposed to us, _under
these circumstances_ is to be looked upon as an enemy’s corps in the
middle of our camp, which, protected by its walls, is _invulnerable_,
or at least not to be overpowered, by which its power is immensely
increased.
3. The defence of a line of circumvallation admits of nothing but the
most absolute defensive, because the circular order, facing outwards,
is the weakest and most disadvantageous of all possible orders of
battle, and is particularly unfavourable to any advantageous
counter-attacks. There is no alternative, in fact, but to defend
ourselves to the last extremity within the entrenchments. That these
circumstances may cause a greater diminution of the army than one-third
which, perhaps, would be occasioned by forming an army of observation,
is easy to conceive. If, added to that, we now think of the general
preference which has existed since the time of Frederick the Great for
the offensive, as it is called, (but which, in reality, is not always
so) for movements and manœuvres, and the aversion to entrenchments, we
shall not wonder at lines of circumvallation having gone quite out of
fashion. But this weakening of the tactical resistance is by no means
its only disadvantage; and we have only reckoned up the prejudices
which forced themselves into the judgment on the lines of
circumvallation next in order after that disadvantage, because they are
nearly akin to each other. A line of circumvallation only in reality
covers that portion of the theatre of war which it actually encloses;
all the rest is more or less given up to the enemy if special
detachments are not made use of to cover it, in which way the very
partition of force which it was intended to obviate takes place. Thus
the besieging army will be always in anxiety and embarrassment on
account of the convoys which it requires, and the covering the same by
lines of circumvallation, is not to be thought of if the army and the
siege supplies required are considerable, and the enemy is in the field
in strong force, unless under such conditions as are found in the
Netherlands, where there is a whole system of fortresses lying close to
each other, and intermediate lines connecting them, which cover the
rest of the theatre of war, and considerably shorten the lines by which
transport can be affected. In the time of Louis the Fourteenth the
conception of a theatre of war had not yet bound itself up with the
position of an army. In the Thirty Years’ War particularly, the armies
moved here and there sporadically before this or that fortress, in the
neighbourhood of which there was no enemy’s corps at all, and besieged
it as long as the siege equipment they had brought with them lasted,
and until an enemy’s army approached to relieve the place. Then lines
of circumvallation had their foundation in the nature of circumstances.
In future it is not likely they will be often used again, unless where
the enemy in the field is very weak, or the conception of the theatre
of war vanishes before that of the siege. Then it will be natural to
keep all the forces united in the siege, as a siege by that means
unquestionably gains in energy in a high degree.
The lines of circumvallation in the reign of Louis XIV., at Cambray and
Valenciennes, were of little use, as the former were stormed by
Turenne, opposed to Condé, the latter by Condé opposed to Turenne; but
we must not overlook the endless number of other cases in which they
were respected, even when there existed in the place the most urgent
need for relief; and when the commander on the defensive side was a man
of great enterprise, as in 1708, when Villars did not venture to attack
the allies in their lines at Lille. Frederick the Great at Olmütz,
1758, and at Dresden, 1760, although he had no regular lines of
circumvallation, had a system which in all essentials was identical; he
used the same army to carry on the siege, and also as a covering army.
The distance of the Austrian army induced him to adopt this plan at
Olmütz, but the loss of his convoy at Domstädtel made him repent it; at
Dresden in 1760 the motives which led him to this mode of proceeding,
were his contempt for the German States’ imperial army, and his desire
to take Dresden as soon as possible.
Lastly, it is a disadvantage in lines of circumvallation, that in case
of a reverse it is more difficult to save the siege train. If a defeat
is sustained at a distance of one or more days’ march from the place
besieged, the siege may be raised before the enemy can arrive, and the
heavy trains may, in the mean time, gain also a day’s march.
In taking up a position for an army of observation, an important
question to be considered is the distance at which it should be placed
from the besieged place. This question will, in most cases, be decided
by the nature of the country, or by the position of other armies or
corps with which the besiegers have to remain in communication. In
other respects, it is easy to see that, with a greater distance, the
siege is better covered, but that by a smaller distance, not exceeding
a few miles, the two armies are better able to afford each other mutual
support.