text: (Principles of Field Fortification) was published by OHL in January 1917 and by April an outpost zone () held by sentries, had been built along the Western Front. Sentries could retreat to larger positions () held by  (five men and an NCO per ), who would join the sentries to recapture sentry-posts by immediate counter-attack. Defensive procedures in the battle zone were similar but with bigger units. The front trench system was the sentry line for the battle zone garrison, which was allowed to move away from concentrations of enemy fire and then counter-attack to recover the battle and outpost zones; such withdrawals were envisaged as occurring on small parts of the battlefield which had been made untenable by Allied artillery fire, as the prelude to  (immediate counter-attack within the position). Such a decentralised battle by large numbers of small infantry detachments would present the attacker with unforeseen obstructions. Resistance from troops equipped with automatic weapons, supported by observed artillery fire, would increase the further the advance progressed. A school was opened in January 1917 to teach infantry commanders the new methods.
  Given the growing Allied superiority in munitions and manpower, attackers might still penetrate to the second  (artillery protection line), leaving in their wake German garrisons isolated in resistance nests  () still inflicting losses and disorganisation on the attackers. As the attackers tried to capture the  and dig in near the German second line,  and  of the counter-attack divisions would advance from the  into the battle zone, in an immediate counter-attack (). If the immediate counter-attack failed, counter-attack divisions would take their time to prepare a methodical attack if the lost ground was essential to the retention of the main position. Such methods required large numbers of reserve divisions ready to move to the battlefront. The reserve was obtained by creating  by internal reorganisation of the army, bringing divisions from the eastern front and by shortening the Western Front, in Operation Alberich. By the spring of 1917, the German army in the west had a strategic reserve of