After the Battle of the Somme (1 July – 18 November), General Erich Ludendorff had German defensive doctrine revised. On 1 December, the (OHL, supreme army command) published new tactical instructions, (Principles of Command for Defensive Battle in Positional Warfare), in which the policy of unyielding defence of ground regardless of its tactical value, was replaced. Positions suitable for artillery observation and communication with the rear were to be defended, where an attacking force would "fight itself to a standstill and use up its resources while the defenders conserve[d] their strength". Defending infantry would fight in areas, with the front divisions in an outpost zone up to deep behind listening posts, with the main line of resistance placed on a reverse slope, in front of artillery observation posts kept far enough back to retain observation over the outpost zone. Behind the main line of resistance was a (main battle zone), a second defensive area deep, also sited as far as possible on ground hidden from enemy observation, while in view of German artillery observers was to be built. A (rear battle zone) further back was to be occupied by the reserve battalion of each regiment.