We have just seen how the security of the army is expected, from the effect which an advanced guard and flank corps produce on an advancing enemy. Such corps are always to be considered as very weak whenever we imagine them in conflict with the main body of the enemy, and therefore a peculiar mode of using them is required, that they may fulfil the purpose for which they are intended, without incurring the risk of the serious loss which is to be feared from this disproportion in strength. The object of a corps of this description, is to observe the enemy, and to delay his progress. For the first of these purposes a smaller body would never be sufficient, partly because it would be more easily driven back, partly because its means of observation that is its eyes could not reach as far. But the observation must be carried to a high point; the enemy must be made to develop his whole strength before such a corps, and thereby reveal to a certain extent, not only his force, but also his plans. For this its mere presence would be sufficient, and it would only be necessary to wait and see the measures by which the enemy seeks to drive it back, and then commence its retreat at once. But further, it must also delay the advance of the enemy, and that implies actual resistance. Now how can we conceive this waiting until the last moment, as well as this resistance, without such a corps being in constant danger of serious loss? Chiefly in this way, that the enemy himself is preceded by an advanced guard, and therefore does not advance at once with all the outflanking and overpowering weight of his whole force. Now, if this advance guard is also from the commencement superior to our advanced corps, as we may naturally suppose it is intended it should be, and if the enemy’s main body is also nearer to his advanced guard than we are to ours, and if that main body, being already on the march, will soon be on the spot to support the attack of his advanced guard with all his strength, still this first act, in which our advanced corps has to contend with the enemy’s advanced guard, that is with a force not much exceeding its own, ensures at once a certain gain of time, and thus allows of our watching the adversary’s movements for some time without endangering our own retreat.