=Principles of the Plan of Defense.= (_a_) The most important principle of organization of a position is the =disposition of troops in depth=. It is necessary to have plenty of support and reserve units disposed one behind the other. Only one-sixth to one-third of a garrison in the support point of the first line remains in the trenches for the purpose of observation and security. The rest remain in the rear as a reserve for reinforcements or counterattacks. (_b_) Each trench, each nest of resistance, each area of the ground, must have a responsible leader. =The garrison of any such area must never leave the ground allotted to its defense under any conditions of battle.= The defense must be carried out by this garrison on the lines originally laid out. A leader is not allowed to fall back on troops in his rear or to take up a better position. The troops in depth are placed there for the purpose of reinforcements for the lines in front. (_c_) =Each part of a position that is lost to the enemy must be immediately retaken by a counterattack= carried out by troops held in reserve. The first line is retaken by reserve troops in the support line, and similarly, if the support line is lost, it must be retaken by reserve troops held in the third line. If all three lines are captured, an attempt must be made to retake the third line by the reserve troops of the sector. =Form of Plan of Defense.= In each division of the position from front to rear there must be a Plan of Defense. This plan always contains the five following elements: 1. Directions of the probable attacks. 2. Defense of the first line, or line of resistance. The outline of the first line. Location and mission of the troops in the first line. Employment of machine guns. 3. Counterattacks. Units which are used for the purpose. Directions or trenches to be used in the counterattack. Depots of grenades and sandbags for the counterattack. 4. Artillery support. Composition of this artillery. Establishing of the front of barrage. Infantry-artillery liaison. 5. Liaisons and supply during the hostile attack. Details of Trench Duty =Guard Duties (Security).= Security is obtained by the use of sentinels and patrols. =Sentinels= must not be confused with observers, watchers, or snipers. Observers belong to the information service of the sector and are not on duty at night. Snipers operate only in the daytime. The watchers of the different lines give the alarm in case of an attack, and are on duty both day and night. Sentinels are posted in the first line or in advanced posts. Their work and number will depend upon the proximity of the enemy, the tactical situation, and above all upon the conditions of the wire entanglements out in front.