1. A LARGE river which crosses the direction of the attack is always very inconvenient for the assailant: for when he has crossed it he is generally limited to one point of passage, and, therefore, unless he remains close to the river he becomes very much hampered in his movements. Whether he meditates bringing on a decisive battle after crossing, or may expect the enemy to attack him, he exposes himself to great danger; therefore, without a decided superiority, both in moral and physical force, a general will not place himself in such a position.
2. From this mere disadvantage of placing a river behind an army, a river is much oftener capable of defence than it would otherwise be. If we suppose that this defence is not considered the only means of safety, but is so planned that even if it fails, still a stand can be made near the river, then the assailant in his calculations must add to the resistance which he may experience in the defence of the river, all the advantages mentioned in No. 1, as being on the side of the defender of a river, and the effect of the two together is, that we usually see generals show great respect to a river before they attack it if it is defended.
3. But in the preceding book we have seen, that under certain conditions, the real defence of a river promises right good results; and if we refer to experience, we must allow that such results follow in reality much more frequently than theory promises, because in theory we only calculate with real circumstances as we find them take place, while in the execution, things commonly appear to the assailant much more difficult than they really are, and they become therefore a greater clog on his action.
Suppose, for instance, an attack which is not intended to end in a great solution, and which is not conducted with thorough energy, we may be sure that in carrying it out a number of little obstacles and accidents, which no theory could calculate upon, will start up to the disadvantage of the assailant, because he is the acting party, and must, therefore, come first into collision with such impediments. Let us just think for a moment how often some of the insignificant rivers of Lombardy have been successfully defended!—If, on the other hand, cases may also be found in military history, in which the defence of rivers has failed to realise what was expected of them, that lies in the extravagant results sometimes looked for from this means; results not founded in any kind of way on its tactical nature, but merely on its well-known efficacy, to which people have thought there were no bounds.
4. It is only when the defender commits the mistake of placing his entire dependence on the defence of a river, so that in case it is forced he becomes involved in great difficulty, in a kind of catastrophe, it is only then that the defence of a river can be looked upon as a form of defence favourable to the attack, for it is certainly easier to force the passage of a river than to gain an ordinary battle.
5. It follows of itself from what has just been said that the defence of a river may become of great value if no great solution is desired, but where that is to be expected, either from the superior numbers or energy of the enemy, then this means, if wrongly used, may turn to the positive advantage of the assailant.
6. There are very few river-lines of defence which cannot be turned either on the whole length or at some particular point. Therefore the assailant, superior in numbers and bent upon serious blows, has the means of making a demonstration at one point and passing at another, and then by superior numbers, and advancing, regardless of all opposition, he can repair any disadvantageous relations in which he may have been placed by the issue of the first encounters: for his general superiority will enable him to do so. It very rarely happens that the passage of a river is actually tactically forced by overpowering the enemy's principal post by the effect of superior fire and greater valour on the part of the troops, and the expression, forcing a passage is only to be taken in a strategic sense, in so far that the assailant by his passage at an undefended or only slightly defended point within the line of defence, braves all the dangers which, in the defender's view, should result to him through the crossing.—But the worst which an assailant can do, is to attempt a real passage at several points, unless they lie close to each other and admit of all the troops joining in the combat; for as the defender must necessarily have his forces separated, therefore, if the assailant fractions his in like manner, he throws away his natural advantage. In that way Bellegarde lost the battle on the Mincio, 1814, where by chance both armies passed at different points at the same time, and the Austrians were more divided than the French.
7. If the defender remains on this side of the river, it necessarily follows that there are two ways to gain a strategic advantage over him: either to pass at some point, regardless of his position, and so to outbid him in the same means, or to give battle. In the first case, the relations of the base and lines of communications should chiefly decide, but it often happens that special circumstances exercise more influence than general relations; he who can choose the best positions, who knows best how to make his dispositions, who is better obeyed, whose army marches fastest, etc., may contend with advantage against general circumstances. As regards the second means, it presupposes on the part of the assailant the means, suitable relations, and the determination to fight; but when these conditions may be presupposed, the defender will not readily venture upon this mode of defending a river.
8. As a final result, we must therefore give as our opinion that, although the passage of a river in itself rarely presents great difficulties, yet in all cases not immediately connected with a great decision, so many apprehensions of the consequences and of future complications are bound up with it, that at all events the progress of the assailant may easily be so far arrested that he either leaves the defender on this side the river, or he passes, and then remains close to the river. For it rarely happens that two armies remain any length of time confronting one another on different sides of a river.
But also in cases of a great solution, a river is an important object; it always weakens and deranges the offensive; and the most fortunate thing, in this case is, if the defender is induced through that to look upon the river as a tactical barrier, and to make the particular defence of that barrier the principal act of his resistance, so that the assailant at once obtains the advantage of being able to strike a decisive blow in a very easy manner.—Certainly, in the first instance, this blow will never amount to a complete defeat of the enemy, but it will consist of several advantageous combats, and these bring about a state of general relations very adverse to the enemy, as happened to the Austrians on the Lower Rhine, 1796.