To ensure the security of the flanks of a deployment, a commander must create a front line of units should be as long as possible, while maintaining a capable re serve; approximately one-third of a commander's available units should be assigned to reserve roles, both for offensive and defensive operations. These formations, being secluded from the direct assault of enemy firepower, must be placed in posi tions whereby their movement will not be impeded by terrain or friendly forma tions. Easy travel over intervening terrain increases the formation's rate of movement - moving faster will allow units to transit larger distances in the same amount of time - thereby allowing reserves to be placed at greater distances from one another.

The primary role of defensive reserves is that of stopping and containing attacking formations. Reserves should be dispersed on a frontage equal to that of front line formations. The placement of reserves too near the center of the position will prevent their support of the flanks of the position; placement of reserves outside of the front line will prevent their support of the interior of the defensive position.

The proper placement of defensive reserves will decrease the amount of time re quired to respond to the point of attack and prevent the outflanking of the front line. Distributing the reserves uniformly will provide for a timely response to any position in the front line. The responsibilities of these reserves may include the further lengthening of the front line (to prevent the outflanking of the front line), defensive reinforcement at the point of attack (to contain the opponent's attack), the replacement of casualties in the front ranks, and the conducting of a counterattack (to drive the opponent back to their starting point).

The placement and the responsibilities of offensive reserves differs substantially from that of defensive reserves. Offensive reserves are massed at the point of attack, to gain a superiority of numbers at the decisive time and place on a battle field, to develop sufficient mass to breakthrough defenses, to exploit an existing opening in the defense, and to cover the reorganization of a successful attack or the withdrawal of one that is unproductive.

The lack of control exhibited by British cavalry was frequently repeated in the Napoleonic era: Waterloo, Campo Mayor, Maguilla. The charge of the Union Brigade at Waterloo is a prime example of the consequences of failing to provide an adequate offensive reserve. The 1,300 men of the Union Brigade counterattacked the initial French assualt, destroying as an effective fighting force the 13,000 men of the French I Corps, taking 2,000 of them prisoner. However, the failure to assign a reserve for the charge resulted in there being no covering formation when the French counter-counterattacked, causing the loss of over half the brigade. Not a bad exchange, but one that could have been avoided if proper attention had been made to the assignment of a reserve.

A cavalry component in the reserve is essential, particularly so for the victor, for without a cavalry reserve no pursuit can take place after the battle. In large part, battles are essentially indecisive during the actual combat, it is during the pursuit where the disproportionate casualties are inflicted. Infantry, not being noted for their speed, do not perform well in a pursuing role; cavalry, being more mobile, are needed in numbers in both offensive and defensive reserves. Both light and heavy cavalry are trained to maneuver at the same speed on the battlefield, to facilitate their cooperation, but the light cavalry have an unused capacity for a superior rate of movement that is not tapped on the battlefield.

Heavy cavalry, having an advantage of weight of horse, excel in personal combat; light cavalry thrive in reconnaissance and pursuit, where speed of horse and endurance are of greater importance. Therefore, reserve heavy cavalry are committed to determine which side wins the battle, and reserve light cavalry to pursue the vanquished.

An officer's best and most mobile formations should be placed in reserves. The reserves should engage the enemy after they experience combat with your front line units, attacking them while they are disordered from prior combat. The initial combats will take place between these opposing front line formations - the 'superior' units, in reserve, then attack the weakened and less skillful opponent. The reserve units, intending to overwhelm their opponent, have an advantage of skill, organization, and esprit de corp. Higher echelon officers, brigadiers and division command ers, should accompany these formations, to ensure their timely maneuvering to the point of attack.

The side with the last sizable reserve is commonly the victor in any period and the Napoleonic Era is no exception. The reasoning is simple; the losing side continually expends his reserves in an effort to contain the opponent, while the victorious side has an uncommitted reserve that can be used to break and pursue the defeated opponent. Therefore, the efficient and timely commitment of reserves is essential for the retention of these decisive reserve elements.

The assignment of one-third of available units to a reserve is a traditional compromise between the mutually exclusive needs of length of frontage and the requirement for an adequate reserve, reserves being required for both offensive and defensive operations. The placement of reserves should be concentrated in the attack and dispersed on the defense, and in a location where they can easily move in all directions; the placement must be unrestricted by terrain or friendly formations - to expedite their response to the point of attack. Pursuit by a reserve of cavalry completes the destruction of a defeated foe.

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