At the same time, the different troops held in reserve in the successive lines are used to hold their own line, to reinforce the line in front, or to make counterattacks. When the alarm is given, all supporting reserve troops take their places in their own lines. In each support point, the platoons held in reserve are sent forward as reinforcements to, or to make counterattacks against, the firing line. Similarly, in each center of resistance the companies held in reserve are employed as reinforcements or to recapture the first line. If, upon arriving at the first line, these troops receive no orders for particular duty, they replace the reserve platoons of the support point. Likewise, the sector reserve of the third line is sent forward to or, in case the enemy has taken it, against the second line. The static or passive organization of the defense lies in the series of lines of trenches disposed in depth, but the active rôle of the defender is carried out in these successive waves of defense moving forward in definite limits to meet the attack, not only to repulse it but to inflict as much loss as possible to the enemy. This transforms the defensive combat into more nearly a fight in the open ground, with the spirit of the offensive. =The Counterattack.= It follows that the last and, very often, the most decisive means of defense of the sector is the counterattack. In each center of resistance, a counterattack has for its purpose the retaking of the first line when this is captured by the enemy. It is made by the battalion reserves located in the reserve line. The sooner the counterattack is made after its necessity, the more effective it will be. It should arrive at the first line almost, one might say, at the same time that the enemy does. At any rate, the counterattack should strike the enemy by surprise while he is still in a state of disorganization. Consequently, in the plan of defense of the battalion, the counterattack must be foreseen and all its details must be laid down. All these details must be known and practiced by the troops who are to execute the counterattack, so that when the alarm is given the counterattack will start automatically without orders and be carried to a finish like a good piece of team-work. The counterattack is launched on the initiative of the leader of these troops, without waiting for the orders of the battalion commander. The latter may not know the exact situation nor be so located in the area as to appreciate the opportunity of the counterattack.