Now, whatever may be the nature of the circumstances which make it easier to cross a river at particular points, they must have an influence on the position of the troops, and modify the general geometrical law; but it is not advisable to deviate too far from that law, relying on the difficulties of the passage at many points. The enemy would choose exactly those spots which are the least favourable by nature for crossing, if he knew that these are the points where there is the least likelihood of meeting us.
In any case the strongest possible occupation of islands is a measure to be recommended, because a serious attack on an island indicates in the surest way the intended point of passage.
As the corps stationed close to a river must be able to move either up or down along its banks according as circumstances require, therefore if there is no road parallel to the river, one of the most essential preparatory measures for the defence of the river is to put the nearest small roads running in a parallel direction into suitable order, and to construct such short roads of connection as may be necessary.
The second point on which we have to speak, is the removal of the means of crossing.—On the river itself the thing is no easy matter, at least requires considerable time; but on the affluents which fall into the river, particularly those on the enemy’s side, the difficulties are almost insurmountable, as these branch rivers are generally already in the hands of the enemy. For that reason it is important to close the mouths of such rivers by fortifications.
As the equipment for crossing rivers which an enemy brings with him, that is his pontoons, are rarely sufficient for the passage of great rivers, much depends on the means to be found on the river itself, its affluents, and in the great towns adjacent, and lastly, on the timber for building boats and rafts in forests near the river. There are cases in which all these circumstances are so unfavourable, that the crossing of a river is by that means almost an impossibility.