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. * * * * *