the river is behind the army, the less useful it will be, and at certain distances its influence disappears altogether.
If an advancing army has to leave a river in its rear, the river cannot be otherwise than prejudicial to its movements, for it restricts the communications of the army to a few single passages. When Prince Henry marched against the Russians on the right bank of the Oder near Breslau, he had plainly a point d’appui in the Oder flowing behind him at a day’s march; on the other hand, when the Russians under Cznernitschef passed the Oder subsequently, they were in a very embarrassing situation, just through the risk of losing their line of retreat, which was limited to one bridge.
If a river crosses the theatre of war more or less at a right angle with the strategic front, then the advantage is again on the side of the defensive; for, in the first place, there are generally a number of good positions leaning on the river, and covered in front by the transverse valleys connected with the principal valley (like the Elbe for the Prussians in the Seven Years’ War); secondly, the assailant must leave one side of the river or the other unoccupied, or he must divide his forces; and such division cannot fail to be in favour again of the defensive, because he will be in possession of more well secured passages than the assailant. We need only cast a glance over the whole Seven Years’ War, to be convinced that the Oder and Elbe were very useful to Frederick the Great in the defence of his theatre of war (namely Silesia, Saxony and the Mark), and consequently a great impediment to the conquest of these provinces by the Austrians and Russians, although there was no real defence of those rivers in the whole Seven Years’ War, and their course is mostly, as connected with the enemy, at an oblique or a right angle rather than parallel with the front.
It is only the convenience of a river as a means of transport, when its course is more or less in a perpendicular direction, which can, in general,