(2) Construct barbed wire entanglements.
(3) Observatory and dugouts.
(4) Trenches.
CHAPTER III
SECTOR AND TRENCH DUTIES
I. Plan of Defense
Definition. The Plan of Defense contains all the different duties necessary to make a proper defense of the position. The execution of these duties is the execution of the Plan of Defense, which follows logically and depends directly upon the Plan of Organization.
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: