Meanwhile, the right-hand body, under Grouchy, was to fall upon the extremity of the Prussian line and overwhelm it.

Such an action against the head of the long Prussian cordon could lead, as the Emperor thought, to but one of two results: either the great majority of the Prussian force, coming up to retrieve this first disaster, would be defeated in detail as it came; or, more probably, finding itself cut off from all aid on the part of Wellington’s forces to the west and its head crushed, the long Prussian line would roll up backwards upon its communications towards the east, whence it had come.

In either case the prime object of Napoleon’s sudden move would have been achieved; and, with the body upon the left, under Ney, pushing up the Brussels road, the body upon the right, under Grouchy, pushing back the head of the Prussian line eastward, the two halves of the Allies would be separated altogether, and could later be dealt with, each in turn. The capital disadvantage under which Napoleon suffered—the fact that he had little more than half as many men as his combined enemies—would be neutralised, because he would, after the separation of those enemies into two bodies, be free to deal with either at his choice. Their communications came from diametrically opposite directions,[5] and, as the plan of each depended upon the co-operation of the other, their separation would leave them confused and without a scheme.

Napoleon in all this exaggerated the facility of the task before him; but before we go into that, it is essential that the reader should grasp a certain character in all military affairs, to misunderstand which is to misread the history of armies.

This characteristic is the necessary uncertainty under which every commander lies as to the disposition, the number, the order, and the information of his opponents.

It is a necessary characteristic in all warfare, because it is a prime duty in the conduct of war to conceal from your enemy your numbers, your dispositions, and the extent of your information. It is a duty which every commander will always fulfil to his best ability.

It is therefore a characteristic, be it noted, which no development of human science can conceivably destroy, for with every advance in our means of communicating information we advance also in our knowledge of the means whereby the new means of communication may be interrupted. An advantage over the enemy in the means one has of acquiring knowledge with regard to him must, of course, always be of supreme importance, and when those means are novel, one side or the other is often beforehand for some years with the new science of their use. When such is the case, science appears to uninstructed opinion to have changed this ancient and fixed characteristic which is in the very nature of war. But in fact there has been no such change. Under the most primitive conditions an advantage of this type was of supreme importance; under conditions the most scientific and refined it is an advantage that may still be neutralised if the enemy has learnt means of screening himself as excellent as our means of discovering him. Even the aeroplane, whose development in the modern French service has so vastly changed the character of information, and therefore of war, can never eliminate the factor of which I speak. A service possessed of a great superiority in this new arm will, of course, be the master of its foe; but when the use of the new arm is spread and equalised among all European forces so that two opposing forces are equally matched even in this new discovery, then the old element of move and countermove, feint, secrecy, and calculated confusion of an adversary, will reappear.[6]

In general, then, to point out the ignorance and the misconceptions of one commander is no criticism of a campaign until we have appreciated the corresponding ignorance and misconceptions of the other. We have already seen Wellington taken almost wholly by surprise on the French advance; we shall see him, even when he appreciated its existence, imagining it to be directed principally against himself. We shall similarly see Napoleon underestimating the Prussian force in front of him, and underestimating even that tardy information which had reached Wellington in time for him to send troops up the Brussels road, and to check the French advance along it. But we must judge either of the two great opponents not by a single picture of his own misconceptions alone, but by the combined picture of the misconceptions of both, and especially by a consideration of the way in which each retrieved or attempted to retrieve the results of those misconceptions when a true idea of the enemy’s dispositions was conveyed to him.