CHOOSING A POSITION.--The framework of the modern defence consists of artillery and machine guns; into this framework are fitted the defence posts or defended localities garrisoned by infantry, who are responsible for holding their ground at all costs and for inflicting the greatest possible loss on the enemy. A commander will require a position which affords elasticity for increasing the resistance as the attackers penetrate the defences, and depth will thus be essential. He will require a position wide enough to prevent the whole of his front being masked by a retaining attack of a part of the {84} enemy's forces while a strong flank attack is simultaneously delivered; and in a War of Manoeuvre he will require facilities for the Decisive Counter-Attack. The depth of the position will develop automatically in a War of Position, but it must always be sufficient to enable troops to assemble in rear of the forward position before moving up and to afford rest to troops when withdrawn from the front line. The width of the position will generally depend upon the strength of the defending force, the guiding principle being to keep about half the force in General Reserve; if, therefore, the remainder of the force is insufficient for the purpose of holding the defences the position is too wide for the tactical requirements of the Active Defence. In Position Warfare, however, a defensive system must necessarily be extended beyond the limits that are practicable in the Active Defence, and the numbers available for the garrison are supplemented by denying ground to the attack by means of obstacles, the removal of which is prevented by machine-gun and rifle fire. THE OUTPOST ZONE.--For the Active Defence of a position the defensive system will consist of an Outpost Zone and a Battle Position. The Outpost Zone is garrisoned by a protective force which keeps a constant watch on the enemy and absorbs 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)).