WE have still to add something respecting the influence of streams and rivers on the defence of a country, even when they are not themselves defended.
Every important river, with its main valley and its adjacent valleys, forms a very considerable obstacle in a country, and in that way it is, therefore, advantageous to defence in general; but its peculiar influence admits of being more particularly specified in its principal effects.
First we must distinguish whether it flows parallel to the frontier, that is, the general strategic front, or at an oblique or a right angle to it. In the case of the parallel direction we must observe the difference between having our own army or that of the enemy behind it, and in both cases again the distance between it and the army.
An army on the defensive, having behind it a large river within easy reach (but not less than a day's march), and on that river an adequate number of secure crossings, is unquestionably in a much stronger situation than it would be without the river; for if it loses a little in freedom of movement by the requisite care for the security of the crossings, still it gains much more by the security of its strategic rear, that means chiefly of its lines of communication. In all this we allude to a defence in our own country; for in the enemy's country, although his army might be before us, we should still have always more or less to apprehend his appearance behind us on the other side of the river, and then the river, involving as it does narrow defiles in roads, would be more disadvantageous than otherwise in its effect on our situation. The further 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, be advantageous to the assailant; in that respect it may be so for this reason, that as he has the longer line of communication, and, therefore, the greater difficulty in the transport of all he requires, water carriage may relieve him of a great deal of trouble and prove very useful. The defender, on his side, certainly has it in his power to close the navigation within his own frontier by fortresses; still even by that means the advantages which the river affords the assailant will not be lost so far as regards its course up to that frontier. But if we reflect upon the fact that many rivers are often not navigable, even where they are of no unimportant breadth as respects other military relations, that others are not navigable at all seasons, that the ascent against the stream is tedious, that the winding of a river often doubles its length, that the chief communications between countries now are high roads, and that now more than ever the wants of an army are supplied from the country adjacent to the scene of its operations, and not by carriage from distant parts,—we can well see that the use of a river does not generally play such a prominent part in the subsistence of troops as is usually represented in books, and that its influence on the march of events is therefore very remote and uncertain.