Here we may observe, that this intermediate position should be
reconnoitred, or rather, well entrenched beforehand; for the enemy will
be unable to make an offensive movement against the corps employed in
the siege, until he has beaten the army of observation; and the latter,
under cover of its camp, may always await a favorable opportunity to
attack him in flank or in rear.
Besides, the army which is once entrenched in this manner, has the
advantage of being concentrated; while that of the enemy must act in
detachments, if he wishes to cover his bridge, and watch the movements
of the army of observation, so as to enable him to attack the besieging
corps in its lines, without being exposed to an attempt on his rear, or
being menaced with the loss of his bridge.