If the essence of his duty is to secure and to screen, then, as stated above, extension and subdivision of his command naturally follows. But one must keep clearly before one's own eyes that any concentration for a combat jeopardizes the performances of these tasks, for the concentration for combat necessitates the uncovering of the front, which it is the very object of the enemy to bring about and take advantage of. Further, the wider the front, the less becomes the possibility of timely tactical concentration; hence, under such conditions, one would generally have to renounce the temptation to engage in combat with a positive object, and content one's self with keeping the enemy at a distance, for this purpose availing one's self to the utmost of the advantages the ground may confer, and such support as the reinforced Horse Artillery and mobile Infantry (in vehicles) can afford. When the width of extension becomes very great it will be an absolute mistake to keep back reserves, since by doing so the actual front is weakened, whilst there is no guarantee that the reserve can arrive in time at the threatened point. Further, the wider the front becomes, the greater the difficulty of strategical change of direction. A similar extension of front as that above indicated in this resume of 'Security' duties may also become advisable when engaged on 'Reconnaissance'--for instance, when lengthy obstacles, such as watercourses, mountains, etc., only traversable at bridges, dams, or defiles, have to be surmounted. In such cases, especially when resistance is to be anticipated, it will rarely be possible to count on being able to surmount the obstacle in one single column, since even unimportant forces under such circumstances can oppose a very protracted defence, and neutralize a large numerical superiority. When these difficulties are met with, it will be necessary even in reconnaissance to divide one's force into several columns, as then there is always the chance of successfully piercing the enemy's front at some one point, and thence rolling up his resistance all along the line. Even in open country conditions may be encountered that render the subdivision of one's forces over a considerable extent of front desirable--_i.e._, when it is necessary to find the enemy, and fix the fact of his presence in certain specific districts. As an example, the conduct of the Cavalry after the Battle of Gravelotte (August 18, 1870), when it became imperative to ascertain whether MacMahon's Army was marching to the north-eastward or not. Here it was indispensable to sweep out a wide area. To attempt such tasks with patrols alone is unadvisable, for these always require a certain tactical support behind them, by which they can be either reinforced or relieved.