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easily countered. Direct attack from the air, therefore, is not likely to affect design seriously. But observation from the air is another matter. Local superiority in the air may pass, in the course of the operations, back and forth several times, and design has to provide against the unfavourable case. It may be assumed with certainty that, in prolonged operations (such as by hypothesis the reduction of a permanently fortified front would entail) every battery posi- tion of the defence would be photographed, whether inside a group or fort or placed in the " interval." The defence is thereby com- pelled either to change position very frequently, or to build in. In the latter case, either the guns must have full fighting protec- tion, or must be so buried and camouflaged as to be practically invisible to eye and to camera at a height of, say 15,000 ft., even when in action, while at the same time provision must be made for confusing the enemy's sound-ranging records. Here, then, the engineer is in the hands of the artillery researcher. Should the lat- ter not succeed in producing gun-mountings, ammunition, and sound-camouflage (so to call it), which can defy the air-observer and the sound-ranging instrument, the engineer will be compelled to put his main armament again under concrete and armour, and perhaps to concentrate it somewhat. The alternative of frequent changes of position has many disadvantages, chief of which perhaps is the probability of having to traverse areas systematically infected by mustard gas.

Armoured Vehicles in the World War were of two classes, the slow-moving cross-country tank and the armoured car. It seems probable that these will fuse into one class, the fast tank. The tank was originally designed for traversing and crushing wire entangle- ments, and it is in this capacity that permanent fortification is chiefly concerned with it, for the defenders of solid permanent works are practically immune from its actual attack. The question for the engineer is will the present types of obstacle-flanking organ succeed in dealing with the tank, and if not, how must they be modi- fied? One type of obstacle, the deep ditch with vertical counter- scarp, is impossible for the unprepared tank, but such ditches are liable to be bridged at ground level by suitable girders carried by a "bridging" tank; and the best safeguard of permanent works against invasion by tanks is the fact that the latter must traverse a belt of ground which is swept at close range by an organ special- ized for that duty alone, and invulnerable to the tank's guns. Al- though the dimensions of tanks will doubtless be taken into account in planning ditches, the first care of the designer will be to provide flanking pieces sufficiently powerful to destroy the tank as it crosses the line of fire, and to provide flanking fire in such a form that a disabled tank will not mask it. More generally, permanent works which form a section of a long battle zone will be organized to par- ticipate in the anti-tank defence of the system, and for this duty the tmditore batteries are naturally the most appropriate organ.

The armoured car, or fast light tank, owing to its ability to make an inroad from a great distance at the shortest notice, may prove very dangerous to a fortified area which is in the crisis of mobiliza- tion and war armament. Special measures to deal with this risk could never be omitted; so far as fortification is concerned it would seem necessary to maintain at all times a clear field of fire for, and a state of readiness in, the traditore batteries, as well as in such gun positions of the safety armament as command obligatory or prob- able points of passage in the foreground.

Field and Heavy Field Fortification. The methods of field forti- fication in vogue at the outbreak of the World War (see 10.719) were concerned essentially with light fortification, that is, incidental fortification on the battlefield. Starting from the conception of an advance brought to a standstill or a retreating army turning to fight, the engineer gave the infantry man types of works which were defensible after an hour's labour and which could be improved gradually during a more prolonged stay in the position. Lateral and overhead cover were provided, but only against rifle, machine- gun and the lightest artillery fire. Great stress was laid upon con- cealment, obtained by moulding the work as closely as possible to the ground, turfing exposed soil, keeping relief as low as conditions of effect allowed, and also upon communications; a system of short trench lengths was preferred to long continuous lines, which entailed unnecessary labour and assisted the enemy to locate the defences. A good field of fire up to 400 yd. from the rifles was considered essen- tial. Pivotal positions, whether localities, woods or hills, were organ- ized for all-round defence. These principles, so long as the general course of the operations retained the foreseen open-warfare char- acter, substantially justified themselves; perhaps, indeed, they justi- fied themselves only too well, since it was the great strength of even lightly fortified fronts which compelled each side, in France, to extend ever further and further to a flank in the hope of turning the opponent's frontal defence, and so in the end produced a trench- line that was continuous from the Swiss frontier to the English Chan- nel. The deadlock which ensued marked the beginning of a position- warfare which was locally indistinguishable from siege warfare as that term was understood in 1914. Nor was it only from the point of view of local, minor tactics that the analogy of siege operations held good. The succeeding campaigns were indeed a siege of the east front of Germany, whose western offensives of 1916-8 have sometimes been described by the Germans themselves as sorties from the Festung Frankreich. For the Allies, as against these xxxn. 16

offensives, the nature of the defence, especially after the middle of 1916, resembled that of an investing cordon attacked by a desperate sortie, and fighting to gain time for the arrival of forces from other parts to drive in the prisoners and bolt the door again, though it is true that the siege was mutual and the " sortie " was at the same time a penetrating assault.

Regarded from this point of view, the principal factors which, directly or indirectly, governed revolution were: (a) a theatre of war richly provided with communications; (6) highly developed industrial resources at the back of both sides; (c) enormous num- bers of men and therefore labour power; (d) an initial situation of tactical deadlock in positions not deliberately selected but repre- senting only the line on which offensive effort in open warfare had expired; (e) the haunting idea of a break-through, followed by exploitation in open-warfare in the old style.

Taking the technical effects of (d) and (e) together, it was inevit- able that positions naturally unsound should be maintained by an equilibrium of forces acting in opposite directions. The attacker of the moment was necessarily attracted by these weak sectors, while the side which intended to attack in the future had a strong motive for holding as forward a position as possible. Fortification was called upon to make these intrinsically unsound positions safe, and all its resources in design and labour were taxed to the utmost to meet the call.

Taking (a), (&) and (c) together, we find these factors enabling each side to produce, to bring up and to apply in situ, technical stores on a scale hitherto undreamed of in siege warfare.

Taking the factors singly (a), by enabling artillery to be collected in enormous strength for an attack, compelled field fortification to develop unimagined powers of passive resistance, and by facilitating the concentration of great attack masses, induced commanders to strip non-offensive sectors of every human defender who could be spared, thereby forcing fortification to adopt forms which required a minimum garrison. By (b) there were brought into current field use not only materials hitherto reserved for permanent work, e.g. concrete, but also novel scientific devices and weapons of war, such as the air camera, gas, tanks, and, above all, improved methods of production, which increased the quantities of guns, ammunition and machine-guns available per unit of living force. By (c) trenches and obstacle-lines were multiplied to such an extent that there were eventually 20 to 25 miles of trench-work for every linear mile of frontage. Through (d), in the first year of trench-warfare, numerous bitter fights were engaged in on all parts of the battle-front for minor rectifications of position; in these fights there were devel- oped new forms of detail tactics, with corresponding demands upon rear resources for a varied armament for the front-line combatant, whose equipment finally comprised, besides rifle and bayonet, grenades (hand and rifle), the light and medium trench mortars, flame throwers, periscopic sniper-rifles, knives, clubs, helmets and body-armour, not to mention gas masks. It led, further, to the retention of waterlogged trenches, and so to the introduction of pumping apparatus, trench-flooring, and other bulky non-com- batant stores, into the front line. Through (c) it was demanded of fortification that it should protect both the masses of men assembled for attack and exposed to enemy " counter-preparation," and also the minimized garrisons of other parts of the front. These factors, operating, singly or together, with different relative intensity at different times, caused position-warfare to evolve, between 1914 and 1918, through three distinct phases, which may be distinguished by the titles field-fortification, position-fortification, and zone- fortification. The first of these characterized the western front from the end of the battle of the Marne to about the end of 1915, and the eastern front from the beginning of stabilization (which occurred at different periods on different parts of the front) to July 1915, and then again from Oct. 15 1915 to the end of the war on that front. On the eastern, Salonika, and extra-European fronts, evolu- tion did not pass beyond this stage, because in these theatres the more important of the factors above enumerated were more or less inoperative. The second stage, position-fortification, is that of the Battles of Verdun, the Somme and the Aisne and the Artois battles of 1917; also of Asiago and the last six of the Isonzp battles on the Italian front. The third stage, zone-fortification, is characteristic of the Flanders battles of the latter half of 1917 and of all the 1918 battles. These indications of date do not of course imply that the changes specified came into force, formally and simultaneously, at any specific time. They are landmarks in the evolutionary process, which in this, as in other things, does not tolerate leaps.

In the first period fortification devotes itself, substantially, to the improvement and multiplication of the trenches bequeathed by the expiring offensives of 1914. The trenches are deepened, kept narrow, provided with more numerous traverses and shelters, though the protection afforded by the last-named only with diffi- culty manages to keep up with the increasing volume of fire from the enemy's medium howitzers. The close ranges at which the 1914 offensives died out have left the opposed front lines separated by a very narrow " no man's land," which is heavily wired by both parties. Concealment, as understood before the war, has thus lost its significance, more especially as all trench systems are regularly photographed from the air. Detail fighting produces the need of circulation and supervision trenches and other parallels behind