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 succor 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 no 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, and lastly, from the actual results of the combats; it is, therefore, an essential condition of strategic command that, according to the degree of uncertainty, forces must be kept in reserve against future contingencies.
In the defensive generally, but particularly in the defence of certain obstacles of ground, like rivers, hills, &c., such contingencies, as is well known, happen constantly.
But this uncertainty diminishes in proportion as the strategic activity has less of the tactical character, and ceases almost altogether in those regions where it borders on politics.
The direction in which the enemy leads his columns to the combat can be perceived by actual sight only; where he intends to pass a river is learnt from a few preparations which are made shortly before; the line by which he proposes to invade our country is usually announced by all the newspapers before a pistol shot has been fired. The greater the nature of the measure the less it will take the enemy by surprise. Time and space are so considerable, the circumstances out of which the action proceeds so public and little susceptible of alteration, that the coming event is either made known in good time, or it can be discovered with certainty.
On the other hand the use of a reserve in this province of strategy, even if one were available, will always be less efficacious the more the measure has a tendency towards being one of a general nature.
We have seen that the decision of a partial combat is nothing in itself, but that all partial combats only find their complete solution in the decision of the total combat.
But even this decision of the total combat has only a relative meaning of many different gradations, according as the force over which the victory has been gained forms a more or less great and important part of the whole. The lost battle of a corps may be repaired by the victory of the army. Even the lost battle of an army may not only be counterbalanced by the gain of a more important one, but converted into a fortunate event (the two days of Culm, 1813). No one can doubt this; but it is just as clear that the weight of each victory (the successful issue of each total combat) is so much the more substantial the more important the part conquered, and that therefore the possibility of repairing the loss by subsequent events diminishes in the same proportion. In another place we shall have to examine this more in detail; it suffices for the present to have drawn attention to the indubitable existence of this progression.
If we now add lastly to these two considerations the third, which is, that if the persistent use of forces in tactics always shifts the great result to the end of the whole act, the law of the simultaneous use of the forces in strategy, on the contrary, lets the principal result (which need not be the final one) take place almost always at the commencement of the great (or whole) act, then in these three results we have grounds sufficient to find strategic reserves always more superfluous, always more useless, always more dangerous the more general their destination.
But the point where the idea of a strategic reserve begins to become inconsistent is not difficult to determine: it lies in the supreme decision. Employment must be given to all the forces within the space of the supreme decision, and every reserve (active force available) which is only intended for use after that decision is opposed to common sense.
If, therefore, tactics has in its reserves the means of not only meeting unforeseen dispositions on the part of the enemy, but also of repairing that which never can be foreseen, the result of the combat, should that be unfortunate; strategy on the other hand must, at least as far as relates to the capital result, renounce the use of these means. As a rule, it can only repair the losses sustained at one point by advantages gained at another, in a few cases by moving troops from one point to another; the idea of preparing for such reverses by placing forces in reserve beforehand, can never be entertained in strategy.
We have pointed out as an absurdity the idea of a strategic reserve which is not to co-operate in the capital result, and as it is so beyond a doubt, we should not have been led into such an analysis as we have made in these two chapters, were it not that, in the disguise of other ideas, it looks like something better, and frequently makes its appearance. One person sees in it the acme of strategic sagacity and foresight; another rejects it, and with it the idea of any reserve, consequently even of a tactical one. This confusion of ideas is transferred to real life, and if we would see a memorable instance of it we have only to call to mind that Prussia in 1806 left a reserve of 20,000 men cantoned in the Mark, under Prince Eugene of Wurtemberg, which could not possibly reach the Saale in time to be of any use, and that another force of 25,000 men belonging to this power remained in East and South Prussia, destined only to be put on a war-footing afterwards as a reserve.
After these examples we cannot be accused of having been fighting with windmills.