s the first shock of the attack, watch
being kept by means of well-concealed sentry posts on the Line of
Observation, supported by a chain of small self-contained defensive
posts, while resistance is offered by a series of self-contained,
mutually supporting defence posts on the Outpost Line of Resistance.

THE BATTLE POSITION.--The Battle Position will be established in the
area in which the commander decides to fight out the battle and break
the enemy's attack. It therefore forms the keystone of the whole
defensive position and must be organised in depth to afford elasticity
for defensive action. "In principle, in order to protect {85} the
battle position from being obliterated by a preliminary bombardment, it
should be beyond effective range of the enemy's mortars" ("Field
Service Regulations," vol. ii. (1920)).

THE SEMI-PERMANENT SYSTEM.--When a campaign is prolonged in any area
without decisive results a War of Position may be developed by one or
both of the combatants. In such cases the Outpost Zone is developed
into an intricate trench system, with protective avenues leading from
front to rear and with deep dugouts to protect the garrison from
artillery fire. The Battle Position will probably coincide with the
Outpost Zone, the trenches being used for the purposes of observation
until the fire positions are manned to resist an assault.

In parts of the line on the Western Front of the Great War, "Pill-box"
forts, constructed of concrete, took the place of continuous lines of
trenches. These machine-gun forts were garrisoned, according to size,
by groups from 5 to 50 strong, and were echeloned in plan, to sweep all
approaches, and together to command with their mutually supporting fire
the whole area over which they were spread, the intervening ground
being entangled with wire so placed as to invite attacking troops into
places where flanking fire may be poured into them. The advantages of
the pill-box system over the continuous line of strong points are
principally defensive. Fewer men are required for them than for the
trench systems, and there is less liability of loss from artillery
fire. But there are certain grave disadvantages. Well-directed
artillery fire is liable to destroy some of the pill boxes, and a
direct hit from a heavy gun will possibly put a larger fort out of
action, thus crippling the defence by the removal of a peg on which the
whole scheme depends. Supports and reserves are nece