Lastly, the fortresses, which lie on both sides, or on the enemy’s side of the river, serve both to prevent any crossing at any points near them, up or down the river, and as a means of closing the mouths of affluents, as well as to receive immediately all craft or boats which may be seized.
So much as to the direct defence of a river, on the supposition that it is one containing a great volume of water. If a deep valley with precipitous sides or marshy banks, are added to the barrier of the river itself, then the difficulty of passing and the strength of the defence are certainly increased; but the volume of water is not made up for by such obstacles, for they constitute no absolute severance of the country, which is an indispensable condition of direct defence.
If we are asked what rôle such a direct river defence can play in the strategic plan of the campaign, we must admit that it can never lead to a decisive victory, partly because the object is not to let the enemy pass over to our side at all, or to crush the first mass of any size which passes; partly because the river prevents our being able to convert the advantages gained into a decisive victory by sallying forth in force.
On the other hand, the defence of a river in this way may produce a great gain of time, which is generally all important for the defensive. The collecting the means of crossing, takes up often much time; if several attempts fail a good deal more time is gained. If the enemy, on account of the river, gives his forces an entirely different direction, then still further advantages may be gained by that means. Lastly, whenever the enemy is not in downright earnest about advancing, a river will occasion a stoppage in his movements and thereby afford a durable protection to the country.
A direct defence of a river, therefore, when the masses of troops engaged are considerable, the river large, and other circumstances favourable, may be regarded as a very good defensive means, and may yield results to