When the whole object of one party is to gain in the end a certain point, and the whole object of his adversary, on the other hand, is to prevent his doing so, then both parties are often obliged to make their movements under the eyes of each other; for this reason, these movements must be made with a degree of precaution and precision not otherwise required. Formerly, before the mass of an army was formed of independent divisions, and even on the march was always regarded as an indivisible whole, this precaution and precision was attended with much more formality, and with the copious use of tactical skill. On these occasions, certainly, single brigades were often obliged to leave the general line of battle to secure particular points, and act an independent part until the army arrived: but these were, and continued, anomalous proceedings; and the aim in the order of march generally was to move the army from one point to another as a whole, preserving its normal formation, and avoiding such exceptional proceedings as the above as far as possible. Now that the parts of the main body of an army are subdivided again into independent bodies, and those bodies can venture to enter into an engagement with the mass of the enemy’s army, provided the rest of the force of which it is a member is sufficiently near to carry it on and finish it,—now such a flank march is attended with less difficulty even under the eye of the enemy. What formerly could only be effected through the actual mechanism of the order of march, can now be done by starting single divisions at an earlier hour, by hastening the march of others, and by the greater freedom in the employment of the whole.
By the means of defence just considered, the assailant can be prevented from taking any fortress, from occupying any important extent of country, or capturing magazines; and he will be prevented, if in every direction combats are offered to him in which he can see little probability of success, or too great danger of a reaction in case of failure, or in general, an expenditure of force too great for his object and existing relations.