But the object of this form of river defence can never be the repulse of a very greatly superior force, as is conceivable in the direct defence of a large river; for as a rule we have really to deal with the bulk of the enemy’s force, and although we do so under favourable circumstances, still it is easy to see the relation between the forces must soon be felt.
This is the nature of the defence of rivers of a moderate size and deep valleys when the principal masses of the armies are concerned, for in respect to them the considerable resistance which can be offered on the ridges or scarps of the valley stands no comparison with the disadvantages of a scattered position, and to them a decisive victory is a matter of necessity. But if nothing more is wanted but the reinforcement of a secondary line of defence which is intended to hold out for a short time, and which can calculate on support, then certainly a direct defence of the scarps of the valley, or even of the river bank, may be made; and although the same advantages are not to be expected here as in mountain positions, still the resistance will always last longer than in an ordinary country. Only one circumstance makes this measure very dangerous, if not impossible: it is when the river has many windings and sharp turnings, which is just what is often the case when a river runs in a deep valley, Only look at the course of the Mosel. In a case of its defence, the corps in advance on the salients of the bends would almost inevitably be lost in the event of a retreat.
That a great river allows the same defensive means, the same form of defence, which we have pointed out as best suited for rivers of a moderate size, in connection with the mass of an army, and also under much more favourable circumstances, is plain of itself. It will come into use more especially when the point with the defender is to gain a decisive victory (Aspern).