roads running parallel to and near a river up and down, are generally better than transverse roads from the interior leading to any particular points on the river. Lastly, the river is unquestionably better watched by corps thus placed than by a mere chain of posts, more particularly as the commanders are all close at hand.—Each of these corps must be concentrated in itself, because otherwise all the calculation as to time would require alteration. He who knows the loss of time in effecting a concentration, will easily comprehend that just in this concentrated position lies the great efficacy of the defence. No doubt, at first sight, it is very tempting to make the crossing, even in boats, impossible for the enemy by a line of posts; but with a few exceptions of points, specially favourable for crossing, such a measure would be extremely prejudicial. To say nothing of the objection that the enemy can generally drive off such a post by bringing a superior force to bear on it from the opposite side, it is, as a rule, a waste of strength, that is to say, the most that can be obtained by any such post, is to compel the enemy to choose another point of passage. If, therefore, we are not so strong that we can treat and defend the river like a ditch of a fortress, a case for which no new precept is required, such a method of directly defending the bank of a river leads necessarily away from the proposed object. Besides these general principles for positions, we have to consider—first, the examination of the special peculiarities of the river; second, the removal of all means of passage; third, the influence of any fortresses situated on the river.
A river, considered as a line of defence, must have at the extremities of the line, right and left, points d’appui , such as, for instance, the sea, or a neutral territory; or there must be other causes which make it impracticable for the enemy to turn the line of defence by crossing beyond its extremities. Now, as neither such flank supports nor such impediments are to be found, unless at considerable distances, we see at once that the defence of a river must embrace a considerable portion of its length, and that, therefore, the possibility of a defence by placing a large body of troops behind a relatively short length of the river vanishes