A reserve has two objects which are very distinct from each other, namely, first, the prolongation and renewal of the combat, and secondly, for use in case of unforeseen events. The first object implies the utility of a successive application of forces, and on that account cannot occur in Strategy. Cases in which a corps is sent to succour a point which is supposed to be about to fall are plainly to be placed in the category of the second object, as the resistance which has to be offered here could not have been sufficiently foreseen. But a corps which is destined expressly to prolong the combat, and with that object in view is placed in rear, would be only a corps placed out of reach of fire, but under the command and at the disposition of the General Commanding in the action, and accordingly would be a tactical and not a strategic reserve.
But the necessity for a force ready for unforeseen events may also take place in Strategy, and consequently there may also be a strategic reserve, but only where unforeseen events are imaginable. In tactics, where the enemy’s measures are generally first ascertained by direct sight, and where they may be concealed by every wood, every fold of undulating ground, we must naturally always be alive, more or less, to the possibility of unforeseen events, in order to strengthen, subsequently, those points which appear too weak, and, in fact, to modify generally the disposition of our troops, so as to make it correspond better to that of the enemy.
Such cases must also happen in Strategy, because the strategic act is directly linked to the tactical. In Strategy also many a measure is first adopted in consequence of what is actually seen, or in consequence of uncertain reports arriving from day to day, or even from hour to hour,