_Field Fortifications_, now more often spoken of as field defences, are
those which are constructed at short notice, with the means locally
available, usually when the enemy is near at hand. Subject to the
question of time, a very high degree of strength can be given to them,
if the military situation makes it worth while to expend sufficient
labour. A century or more ago, the dividing line between permanent and
field fortification was very rigidly drawn, since in those days a high
masonry escarp surmounted by a rampart was essential to a permanent
fortress, and these could naturally not be extemporized. Works without
masonry, in other ways made as strong as possible with deep ditches and
heavy timbers,--such as would require about six weeks for their
construction--were known as _semi-permanent_, and were used for the
defence of places which acquired strategic importance in the course of a
war, but were not immediately threatened. The term _field_ fortification
was reserved for works constructed of lighter materials, with parapets
and ditches of only moderate development. Redoubts of this class
required a fortnight at most for their construction.
In modern fortification if cupolas and deep revetted ditches were
essential to permanent defences, the dividing line would be equally
clear. But as has been shown, this is not universally admitted, and
where the resources exist, the use of our present means of
construction, such as steel joists, railway rails, reinforced concrete
and wire, in conjunction with the defensive power of modern firearms,
makes it possible to extemporize in a very short time works having much
of the resisting power of a permanent fortress. Further, such works can
be expanded from the smallest beginnings; and, if the site is not too
exposed, in the presence of the enemy.
Field fortification offers, as regards the actual constructions, a very
limited scope to the engineer; and a little consideration will show that
its defensive possibilities were not greatly affected by the change from
machine-thrown projectiles to those fired by rude smooth-bore guns.
There is therefore nothing in the history of this branch of the subject
that is worth tracing, from the earliest ages to about the end of the
18th century. One or two points may be noticed. The use of obstacles is
probably one of the earliest measures of defence. Long before missile
weapons had acquired such an importance as to make it worth while to
seek shelter from them, it would obviously have been found desirable to
have some means of checking the onrush of an enemy physically or
numerically superior. Hence the use by savage tribes, to this day, of
pits, pointed stakes hidden in the grass, entanglements and similar
obstacles. In this direction the ages have made no change, and the most
highly civilized nations still use the same obstacles on occasion.
Another use of field defences common to all ages is the protection of
camps at night, where small forces are operating against an enemy more
numerous but inferior in arms and discipline. In daylight such an enemy
is not feared, but at night his numbers might be dangerous. Hence the
Roman practice of making each foot-soldier carry a couple of stakes for
palisades; and the simple defence of a thorn zariba used by the British
for their camps in the Sudan.
Palisades and trenches, abatis and sharpened stakes have always been
used. Except wire, there is practically no new material. As to methods,
the laagers of the Boers are preceded by the wagon-forts of the
Hussites, and those no doubt by similar arrangements of British or
Assyrian war chariots; and so in almost every direction it will be found
that the expedient of to-day has had its forerunners in those of the
countless yesterdays. The only really marked change in the arrangements
of field defences has been caused not by gunpowder but by quick-firing
rifled weapons. For that reason it is worth while to consider briefly
what were the principles of field fortification at the end of the 18th
century. That period has been chosen because it gives us the result of a
couple of centuries of constant fighting between disciplined troops with
fairly effective firearms. The field defences of the 19th century are
transitional in character. Based mainly on the old methods, they show
only faint attempts at adaptation to new conditions, and it was not till
quite the end of the century that the methods now accepted began to take
shape.
The essential elements of fieldworks up to the time of the Peninsular
War were _command_ and _obstacle_; now they are _protection_ and
_concealment_.
Old type of field defences.
The command and obstacle were as necessary in the days of smooth-bore
muskets and guns as in those of javelins and arrows. When the enemy
could get close up to a work without serious loss, and attack in close
order, the defenders needed a really good obstacle in front of them.
Moreover, since they could not rely on their fire alone to repulse the
attack, they needed a two-deep line, with reserves close at hand, to
meet it with the "arme blanche." For this purpose a parapet 7 or 8 ft.
high, with a steep slope, perhaps palisaded, up which the attackers must
climb after passing the obstacle, was excellent. The defenders after
firing their last volley could use their bayonets from the top of the
parapet with the advantage of position. The high parapet had also the
advantage that the attackers could not tell what was going on inside the
redoubt, and the defenders were sheltered from their fire as well from
view until the last moment.
The strength of a fortified line in the 18th century depended
principally on its redoubts. Lines of shelter trenches had little power
of defence at the time, unless they held practically as many men as
would have sufficed to fight in the open. Obstacles on the other hand
had a greater value, against the inelastic tactics of the time, than
they have now. A good position therefore was one which offered good
fire-positions for redoubts and plenty of facilities for creating
obstacles. Strong redoubts which could resist determined assaults; good
obstacles in the intervals, guns in the redoubts to sweep the intervals,
and troops in formed bodies kept in reserve for counter-strokes--these
were the essentials in the days of the smooth-bore.
The redoubts were liable to a heavy cannonade by field-guns before the
attack. To withstand this, the parapets had to be made of a suitable
thickness--from 4 or 5 ft. upwards--according to the time available, the
resisting nature of the soil, and the severity of the bombardment
expected.
The whole of the earth for the parapet was as a rule obtained from the
ditch, in order to make as much as possible of this obstacle. The
garrison in all parts of the interior of the redoubt were to be
sheltered, if possible, from the enemy's fire, and with this object
great pains were bestowed on the principle of "defilade." The object of
defilade, which was a great fetish in theoretical works, was so to
arrange the height of the parapet with reference to the terreplein of a
work that a straight line (not, be it observed, the trajectory of the
projectiles) passing from the muzzle of a musket or gun on the most
commanding point of the enemy's position, over the crest of the parapet,
should just clear the head of a defender standing in any part of the
work. This problem of defilade became quite out of date after the
development of time shrapnel, but was nevertheless taught with great
rigour till within the last twenty years.
The sectional area of the ditch was calculated so that with an addition
of about 10% for expansion it would equal that of the parapet. If a
wider and deeper ditch was considered necessary, the surplus earth could
be used to form a glacis.
The interior of the redoubt had to afford sufficient space to allow the
garrison to sleep in it, which was sometimes a matter of some difficulty
if a small irregularly shaped work had to contain a strong garrison.
Consideration of the plan and sections of these works will show that the
banquette for infantry with its slopes, and the gun platforms, took off
a good deal from the interior space within the crest-line. Guns were
usually placed at the salients, where they could get the widest field of
fire. They were sometimes placed on the ground level, firing through
embrasures in the parapet, and sometimes on platforms so as to fire over
the parapet (_en barbette_).
As in permanent fortification, immense pains were taken to elaborate
theoretically the traces of works. A distinction was made between forts
and redoubts, the former being those which were arranged to flank their
own ditches, while the redoubts did not. Redoubts again were classed as
"closed," those which had an equally strong defence all round; and
"half-closed," those which had only a slight parapet or timber stockade
for the gorge or rear faces. Open works (those which had no gorge
defence) were named according to their trace, as _redans_ and
_lunettes_. A redan is a work with two faces making a salient angle. It
was frequently used in connexion with straight lines of trench or
breastwork. A lunette is a work with two faces, usually forming an
obtuse angle, and two flanks.
The forts described in the text-books, as might be expected, were
designed with great ingenuity, with bastioned or demi-bastioned fronts,
star traces, and so forth, and in the same books intricate calculations
were entered into to balance the _remblai_ and _deblai_, that is, the
amount of earth in the parapets with that excavated from the ditches. In
practice such niceties of course disappeared, though occasionally when
the ground allowed of it star forts and bastioned fronts were employed.
On irregular ground the first necessity was to fit the redoubt to the
ground on which it stood, so as to sweep the whole of the foreground,
and this was generally a sufficiently difficult matter without adding
the complications of flanking defences. Sir John Jones, speaking of the
traces of the several works in the Torres Vedras lines, says:--
Torres Vedras.
"The redoubts were made of every capacity, from that of fig. 74 a,
limited by want of space on the ground it occupied to 50 men and two
pieces of artillery, to that of fig. 74 b, for 500 men and six pieces
of artillery, the importance of the object to be attained being the
only guide in forming the dimensions. Many of the redoubts first
thrown up, even some of the smallest, were shaped like stars, under
the idea of procuring a flank defence for the ditches; but this
construction was latterly rejected, it being found to cut up the
interior space, and to be almost fallacious with respect to flank
defence, the breadth of the exterior slopes being in some cases equal
to the whole length of the flanks so obtained. Even when, from the
greater size of the work, some flanking fire was thus gained, the
angle formed by the faces was generally so obtuse that it demanded
more coolness in the defenders than ought reasonably to be expected to
aim along the ditch of the opposite face: and further, this
construction prevented the fire of the work being more powerful in
front than in rear.
[Illustration: FIG. 74.--Torres Vedras Works.]
In order to decide on the proper trace of a work, it is necessary to
consider whether its object be to prevent an enemy establishing
himself on the ground on which it is to be placed, or whether it be to
insure a heavy fire of artillery on some other point in its vicinity.
In the first case every consideration should be sacrificed to that of
adding to its powers of self-defence by flanks or other expedients. In
the second, its powers of resistance are secondary to the
establishment of a powerful offensive fire and its trace cannot be too
simple. Latterly, the shape of the redoubts was invariably that most
fitted to the ground, or such as best parried the enfilade fire or
musketry plunge of neighbouring heights, care being taken to present
the front of fire deemed necessary towards the pass, or other object
to be guarded; and such will generally be found the best rule of
proceeding.
This recommendation, however, is not intended to apply to isolated
works of large dimensions, and more particularly to those considered
the key of any position. No labour or expense should be spared to
render such works capable of resisting the most furious assaults,
either by breaking the parapet into flanks, or forming a flank defence
in the ditch; for the experience gained in the Peninsula shows that an
unflanked work of even more than an ordinary field profile, if
skilfully and determinedly assaulted, will generally be carried....
Nor does the serious evil of curtailing the interior space, which
renders breaks in the outline so objectionable in small works, apply
to works of large dimensions.... Under this view the great work on
Monte Agraca (fig. 75) must be considered as very defective, the flank
defence being confined to an occasional break of a few feet in the
trace, caused by a change of direction in the contour of the height,
whilst the interior space is more than doubly sufficient for the
number of its allotted garrison to encamp.
[Illustration: FIG. 75.--Monte Agraca, Torres Vedras.]
_Interior and other Defences._--This work, however, had some of its
salient points ... cut off by earthen lines of parapet, steeply
revetted externally, and so traced as to serve for traverses to the
interior. It had also three or four small enclosed posts formed within
it; and the work at Torres Vedras (fig. 76) had each of its salient
points formed into an independent post. These interior defences and
retrenchments were intended to guard against a general panic amongst
the garrison, which would necessarily be composed in part of
indifferent troops, and also to prevent the loss of the work by the
entry of the assailants at any weak or ill-defended point. Such
interior lines to rally on are absolutely essential to the security of
a large field-work. They serve as substitutes for a blockhouse or
tower, placed in the interior of all well-constructed permanent
earthen works, and merit far more attention than they generally
receive.
[Illustration: FIG. 76.--Torres Vedras Works.]
The small circular windmills of stone, which were frequently found
occupying salient knolls ... readily converted into admirable interior
posts of that nature. The profile of the several works varied on every
face and flank, according to its liability to be attacked or
cannonaded; the only general rule enforced being that all ditches
should be at least 15 ft. wide at top and 10 ft. in depth, and the
crest of the parapet have at least 5 ft. command over the crest of the
counterscarp. No parapet exceeded 10 ft. in thickness, unless exposed
to be severely cannonaded, and few more than 6 or 8 ft.; and some, on
high knolls, where artillery could not by any possibility be brought
against them, were made of stone or rubble less than 2 ft. in
thickness, to gain more interior space, and allow full liberty for the
use of the defenders' bayonets."
Fig. 77 gives two typical sections of these works.
[Illustration: FIG. 77.]
The works of Torres Vedras have been chosen for illustration because
they offer very good historical examples, and also because of the value
of the critical remarks of Sir John Jones, who as a captain was the
engineer in charge of their construction. At the same time it must be
remembered that they differ from ordinary field-works in having an
unusual degree of strength, plenty of time and civilian labour having
been available for their construction. In this respect they approximate
more to semi-permanent works, the main reason why they did not receive
under the circumstances a greater development of ditch and parapet being
that in addition to the large number of works required, much labour was
expended in abatis, inundations, scarping hill-sides and constructing
roads.
Some further remarks of Sir John on the _situations of the works_ are
very instructive:--
"Many of the redoubts were placed on very elevated situations on the
summit of steep hills, which gave them a most imposing appearance;
but it was in reality a defect ... for the fire of their artillery on
the object to be guarded became so plunging as to lose half its
powers; the musketry could not be made to scour the face of the hill
sufficiently; and during the night both arms became of most uncertain
effect.
"The domineering situation of the redoubts, however, gave confidence
to the young troops which composed their garrisons, protected them
from a cannonade, and screened their interior from musketry, unless
fired at a high angle, and consequently at random. These
considerations perhaps justify the unusually elevated sites selected
for most of the redoubts on the lines, though they cannot induce an
approval of them as a general measure."
The chief principle of the period was thus that the redoubts were the
most important features of lines of defence, and that they combined
physical obstacle and protection with good musketry and artillery
positions. The value of concealment was not ignored, but it was as a
rule subordinated to other considerations.
19th century.
The principles of this time remained unaltered until after the Crimean
War. In the American Civil War the power of the rifle began to assert
itself, and it was found that a simple breastwork defended by a double
rank of men could protect itself by its fire against an ordinary
assault. This power of the rifle gave greatly enhanced importance to any
defences that could be hastily extemporized behind walls, hedges or any
natural cover. About the period of the Franco-German War other
considerations came in. The increased velocity of artillery projectiles
reduced in some ways their destructive effects against earth parapets,
because the shell had an increasing tendency to deflect upwards on
striking a bank of loose earth. Also the use of shrapnel made it
impossible for troops to find cover on the terreplein of a work some
distance behind the parapet.
These considerations, however, were not fully realized at that time. The
reason was partly a want of touch between the engineers and the
non-technical branches of most armies, and partly that original writers
from the Napoleonic wars to the present day have been more occupied with
the primary question of the value of field defences as a matter of
tactics than with their details considered from the standpoint of
fortification.
There was always an influential school of writers who declaimed against
all defences, as being injurious to the offensive spirit so essential to
success. Those writers who treated of the arrangements of defences
devoted themselves to theoretical details of trace quite after the old
style; discussing the size and shape of typical redoubts, their distance
apart and relation to lines of trenches, &c. The profiles--the thick
parapet with command of 7 ft. or more, the deep ditch, and the
inadequate cover behind the parapet--remained as they had been for a
century.
The American Civil War showed the power of rifles behind slight
defences. Plevna in 1877 taught a further lesson. It proved the great
resisting power of extemporized lines; but more than that, we begin to
find new arrangements for protection against shell fire (see plans and
sections in Greene's _The Russian Army and its Campaign in Turkey_). The
trace of the works and the sections of parapet and ditch suggest Torres
Vedras; but a multiplication of interior traverses and splinter-proof
shelters show the necessity for a different class of protection. The
parapet was designed according to the old type, for want of a better;
the traverses and shelters were added later, to meet the necessities of
the case. The Turks also used two or three tiers of musketry fire, as
for instance one from the crest of the glacis, one from the parapet, and
one from a traverse in rear of it. This, however, is a development which
will not be necessary in future, thanks to magazine rifles.
Principles of modern field defences.
From 1877 to 1899 the efficiency of rifles and guns rapidly increased,
and certain new principles, causing the field defences of the present
day to differ radically from those of the 18th century, remained to be
developed. These may be considered under the following heads: the nature
of protection required, the diminished need of obstacle, and the
adaptation of works to ground.
The principle that _thickness_ of parapet is no longer required, to
resist artillery fire, was first laid down at Chatham in 1896. The
distance at which guns now engage makes direct hits on parapets
comparatively rare. Further, a shell striking near the crest of a
parapet may perhaps kill one man if he is in the way, and displace a
bushel of earth. That is nothing. It is the contents of the shell,
whether shrapnel or explosive, that is the source of danger and not the
shell itself. Thus the enemy's object is to burst his common shell
immediately behind the parapet, or his shrapnel a short distance in
front of it, in order to get searching effect. It follows that a parapet
is thick enough if it suffices to stop rifle bullets, since the same
thickness will _a fortiori_ keep out shrapnel bullets or splinters of
shell. For this purpose 3 ft. is enough.
Real protection is gained by a trench close in rear of the parapet, deep
enough to give shelter from high angle shrapnel, and narrow enough to
minimize the chance of a common shell dropping into it. This protection
is increased by frequent traverses across the trench.
The most essential point of all is _concealment_. In gaining this we say
good-bye finally to the old type of work. Protection is now given by the
trench rather than the parapet; command and the ditch-obstacle (which
furnished the earth for the high parapet) are alike unnecessary.
Concealment can therefore be studied by keeping the parapet down to the
lowest level above the surface from which the foreground can be seen.
This may be 18 in. or less.
The need of obstacle, in daylight and when the defenders are not
abnormally few, has practically disappeared. For night work, or when the
assailant is so strong as to be able to force home his attack in face of
protected rifle fire, what is needed is not a deep ditch immediately in
front of the parapet, difficult to climb, but also difficult to flank,
but an obstacle that will detain him under fire at short range. It may
be an entanglement, an abatis, an inundation: anything that will check
the rush and make him move slowly.
In the _adaptation of works to ground_, the governing factor is the
power of the rifle in frontal defence. We have seen that in Peninsular
times great reliance was placed on the flanking defence of lines by guns
in redoubts. Infantry extended behind a simple line of trench could not
resist a strong attack without such support. Now, however, infantry
behind a slight trench, with a good field of fire should be able to
defend themselves against any infantry attack.
This being so, the enemy's artillery seeks to locate the trenches and to
cover them with a steady hail of shells, so as to force the defenders to
keep down under cover. If they can succeed in doing this, it is possible
for the attacking infantry to advance, and the artillery fire is kept up
until the last moment, so that the attack may have the narrowest
possible space to cover after the defenders have manned their parapets
and opened fire. Fig. 78 shows the action of various natures of
projectiles.
[Illustration: From _Mil. Engineering_, by permission of the Controller
of H.M. Stationery Office.