The sympathetic nervous system mobilizes and directs the body’s energy resources for action. It is the physiological equivalent of the frontline soldiers who actually do the fighting in a military unit.
The parasympathetic system is responsible for the body’s digestive and recuperative processes. It is the physiological equivalent of the cooks, mechanics, and clerks that sustain a military unit over an extended period of time.
Usually these two systems sustain a general balance between their demands upon the body’s resources, but during extremely stressful circumstances the “fight or flight” response kicks in and the sympathetic nervous system mobilizes all available energy for survival. This is the physiological equivalent of throwing the cooks, bakers, mechanics, and clerks into the battle. In combat this very often results in nonessential activities such as digestion, bladder control, and sphincter control being completely shut down. This process is so intense that soldiers very often suffer stress diarrhea, and it is not at all uncommon for them to urinate and defecate in their pants as the body literally “blows its ballast” in an attempt to provide all the energy resources required to ensure its survival.
It doesn’t take a rocket scientist to guess that a soldier must pay a heavy physiological price for an enervating process this intense. The price that the body pays is an equally powerful backlash when the neglected demands of the parasympathetic system become ascendant. This parasympathetic backlash occurs as soon as the danger and the excitement are over, and it takes the form of an incredibly powerful weariness and sleepiness on the part of the soldier.
This brings us to the criticality of the reserve:
Napoleon stated that the moment of greatest danger was the instant immediately after victory, and in saying so he demonstrated a remarkable understanding of the way in which soldiers become physiologically and psychologically incapacitated by the parasympathetic backlash that occurs as soon as the momentum of the attack has halted and the soldier briefly believes himself to be safe. During this period of vulnerability, a counterattack by fresh troops can have an effect completely out of proportion to the number of troops attacking.
It is basically for this reason that the maintenance of an “unblown” reserve has historically been essential in combat, with battles often revolving around which side can hold out and deploy their reserves last. The reserve has always played a vital role in combat, but du Picq was one of the earliest advocates not only of “holding out a reserve as long as possible for independent action when the enemy has used his own,” but he also insisted on the revolutionary concept that this process “ought to be applied downward” to the lowest levels. He also perceived the technological process of increasing lethality on the battlefield which continues today. “There is more need than ever to-day, for protecting… the reserves. The power of destruction increases, the morale [of human beings] stays the same.” Clausewitz further understood and put great emphasis on the danger of reserve forces becoming prematurely enervated and exhausted when he cautioned that the reserves should always be maintained out of sight of the battle.
These same basic psycho-physiological principles explain why successful military leaders have historically maintained the momentum of a successful attack. Pursuing and maintaining contact with a defeated enemy is vital in order to completely destroy the enemy (the vast majority of the killing in historical battles occurred during the pursuit, when the enemy turned his back), but it is also valuable to maintain contact with the enemy as long as possible in order to delay that inevitable pause in the battle which will result in the “culmination point.” The culmination point is usually caused as much by logistical processes as anything else, but once the momentum of the pursuit stops (for whatever reasons) there are severe physiological and psychological costs to be paid, and the commander must realize that his forces will begin to immediately slip into a powerful parasympathetic backlash and become vulnerable to any enemy counterattack. An unblown reserve force ready to complete the pursuit is a vital aspect of maneuver warfare and can be of great value in ensuring that this most destructive phase of the battle is effectively executed.