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The need to revert to the defensive on the Western Front in 1918, following three years of continuous offensive action, prompted the BEF to review and update its defensive doctrine. Convinced by their Third Ypres experience of the efficiency of the German ‘Defence-in-depth’ system the BEF attempted to copy it.
The German ‘Defence-in-depth’ system was based around the following principles:
‘The defender must not surrender the initiative to the attacker.
The defence must rely on firepower, not large numbers of troops.
The defender must not hold ground at all costs.
The defender must consider depth for all construction and positions.’. Timothy T. Lupfer, ‘The Dynamics of Doctrine: The Changes in German Tactical Doctrine during the First World War’, Leavenworth Papers, July 1981, p. 12.
The Germans applied these principles through the use of three successive interrelated zones: the Outpost Zone, the Battle Zone, and the Rearward Zone. The Outpost Zone with a depth varying between 500 and 1000 metres depending on the terrain, comprising a number of lightly held trench lines, was designed to slow an enemy down and provide early warning. This was done by carefully positioning this zone such that the subsequent Battle Zone was hidden, on a reverse slope where possible, and beyond enemy artillery range. The Battle Zone, 1.5 to 3 km in depth, began with a Main Line of Resistance (MLR) that comprised of a series of, usually three, trench lines. Behind the MLR were a number of carefully positioned mutually supporting strong-points arranged in depth. At the rear of the Battle Zone were another series of trench lines called the artillery protection line behind which the Germans located their artillery. Behind the artillery protection line came the Rearward Zone, an area extensively prepared that could be turned into a Battle Zone if required, that was home to the German reserves. The defence principles when applied did not mean that the German soldiers deployed in the Outpost Zone or along the MLR were given authority to evacuate their positions at the first opportunity. They were expected to shift their position, usually into shell holes, to escape artillery fire, and to survive ready to inflict casualties on the subsequent infantry assault. The corner stone of this defence-in-depth system was the counter attack. At every level the principle applied was shift, bend, and snap back, leaders at all levels were given the authority to decide whether to counterattack immediately or to fall back. NCO’s at the squad level were expected to make battlefield decisions, delaying actions in the Outpost Zone, counterattacks in the Battle Zone. Specialist counterattack units were also formed, the stormtroop detachments. Within the Battle Zone as many as 80% of the German infantry were appointed as counterattack forces.
The theory behind the German defence-in-depth was relatively simple. An enemy, with artillery support, would penetrate the Outpost Zone, where they would be delayed and harried. They would then enter the Battle Zone disorganised and unprotected by their artillery to find themselves subjected to German artillery fire and counterattacks launched from both the Battle and Rearward Zones.
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