German Field Fortifications 1939–45

'Elastic defence'
The experiences on the Western Front during World War I had a strong influence (both negative and positive) on post-war defence doctrine. The positive aspect of World War I to the Germans, from the doctrinal standpoint, was the development of what is informally known as the 'elastic defence' (elastische Kampfverfahren, literally 'elastic battle procedures'). By 1916 it had been realised that solid multi-layered trench systems and an unyielding defence, aimed at holding on to every metre of ground, were impractical. Massive six-day artillery barrages would shatter defences and the defenders. General of Infantry Erich Ludendorff endorsed a more in-depth defence. While still relying on continuous interconnected trench lines, the defences were subdivided into three zones: (1) combat outpost zone with minimal lookouts to warn of attacks and keep patrols from penetrating deeper; (2) 1,500-3,000m-deep main battle zone with complex trench systems concentrated on key terrain (rather than rigid lines covering all areas) intended to halt the attack; and (3) rear zone with artillery and reserves. While the battle zone still relied on trench lines, to establish the new defences the Germans actually withdrew (previously unheard of) in some sectors to more easily defended terrain, placed many of the trenches on reverse slopes to mask them from enemy observation and fire, and established strongpoints on key terrain. The establishment of the combat zone, supported by long-range artillery, disrupted Allied attacks. After fighting its way through the outpost zone the attack would often exhaust itself in the battle zone. Rather than attempting to halt the attack outright, penetration of the battle zone was accepted. The attack would become bogged down among the defences, battered by artillery fire and counter-attacks. This was first implemented in April 1917, and by war's end in November 1918 the defences were completely rearranged under this concept. It had proved itself, and was adopted by the post-war Reichsheer in 1921.