This direct defence, as a rule, seems only suitable to large rivers, and on the last half of their course.
The second form of defence is suitable for smaller rivers with deep valleys, often also for very unimportant ones. It consists in a position taken up further back from the river at such a distance that the enemy’s army may either be caught in detail after the passage (if it passes at several points at the same time) or if the passage is made by the whole at one point, then near the river, hemmed in upon one bridge and road. An army with the rear pressed close against a river or a deep valley, and confined to one line of retreat, is in a most disadvantageous position for battle; in the making proper use of this circumstance, consists precisely the most efficacious defence of rivers of moderate size, and running in deep valleys.
The disposition of an army in large corps close to a river which we consider the best in a direct defence, supposes that the enemy cannot pass the river unexpectedly and in great force, because otherwise, by making such a disposition, there would be great danger of being beaten in detail. If, therefore, the circumstances which favour the defence are not sufficiently advantageous, if the enemy has already in hand ample means of crossing, if the river has many islands or fords, if it is not broad enough, if we are too weak, etc., etc., then the idea of that method may be dismissed: the troops for the more secure connection with each other must be drawn back a little from the river, and all that then remains to do is to ensure the most rapid concentration possible upon that point where the enemy attempts to cross, so as to be able to attack him before he has gained so much ground that he has the command of several passages. In the present case the river or its valley must be watched and partially defended by a chain of outposts whilst the army is disposed in several corps at suitable points and at a certain distance (usually a few leagues) from the river.