If we suppose the lines of communication of the assailant, a b, so situated with respect to those of the enemy, c d, that the two lines form a considerable angle with each other, it is evident that if the defensive wishes to take up a position at e, where the two lines intersect, the assailant from b, by the mere geometrical relation, could compel him to form front opposite to him, and thus to lay bare his communications. The case would be reversed if the defensive took up his position on this side of the point of junction, about d; then the assailant must make front towards him, if so be that his line of operations, which closely depends on geographical conditions, cannot be arbitrarily changed, and moved, for instance, to the direction a d. From this it would seem to follow that the defender has an advantage in this system of reciprocal action, because he only requires to take a position on this side of the intersection of the two lines. But very far from attaching any importance to this geometrical element, we only brought it into consideration to make ourselves the better understood; and we are rather of opinion that local and generally individual relations have much more to do with determining the position of the defender; that, therefore, it is quite impossible to lay down in general which of two belligerents will be obliged soonest to expose his communications.
If the lines of communication of both sides lie in one and the same direction, then whichever of the two parties takes up an oblique position will certainly compel his adversary to do the same. But then there is nothing gained geometrically by this, and both parties attain the same advantages and disadvantages.
In the continuation of our considerations we shall, therefore, confine ourselves to the case of the line of communication of one side only being exposed.
Now as regards the second disadvantageous relation of a line of communication, that is to say, when it runs through an enemy’s country, it is clear in itself how much the line is compromised by that circumstance, if the inhabitants of the country have taken up arms; and consequently the case must be looked at as if a body of the enemy was posted all along the line; this body, it is true, is in itself weak without solidity or intensive force; but we must also take into consideration what the close contact and influence of such a hostile force may nevertheless effect through the number of points which offer themselves one after another on long lines of communication. That requires no further explanation. But even if the enemy’s subjects have not taken up arms, and even if there is no militia in the country, or other military organisation, indeed if the people are even very unwarlike in spirit, still the mere relation of the people as subjects to a hostile government is a disadvantage for the lines of communication of the other side which is always felt. The assistance which expeditionary forces and partisans derive merely through a better understanding with the people, through a knowledge of the country and its inhabitants, through good information, through the support of official functionaries, is, for them, of decided value; and this support every such body will enjoy without any special effort on its own part. Added to this, within a certain distance there will not be wanting fortresses, rivers, mountains, or other places of refuge, which of ordinary right belong to the enemy, if they have not been formally taken possession of and occupied by our troops.
Now in such a case as is here supposed, especially if attended with other favourable circumstances, it is possible to act against the communications of an army, although their direction is perpendicular to the position of that army; for the detachments employed for the purpose do not then require to fall back always on their own army, because being in their own country they are safe enough if they only make their escape.
We have, therefore, now ascertained that—
1. A considerable length,
2. An oblique direction,
3. An enemy’s province,