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