As thus defined, the military art may be divided into four distinct branches, viz.:  1st. Strategy; 2d.  Fortification, or Engineering; 3d. Logistics; 4th. Tactics.  Several general treatises on this art add another branch, called The Policy of War, or the relations of war with the affairs of state.

Strategy is defined to be the art of directing masses on decisive points, or the hostile movements of armies beyond the range of each other’s cannon. Engineering embraces all dispositions made to enable troops to resist a superior force the longest time possible; and also the means resorted to by the opposing army to overcome these material obstacles. Logistics embraces the practical details of moving and supplying armies. Tactics is the art of bringing troops into action, or of moving them in the presence of an enemy, that is, within his view, and within the reach of his artillery.  All these are most intimately connected.  A fault in tactics may occasion the loss of strategic lines; the best combined manoeuvres on the field of battle may lead to no decisive results, when the position, or the direction of the operation is not strategic; sometimes not only battles, but entire campaigns, are lost through neglect of the engineer’s art, or faults in his dispositions; again, armies would be of little use without the requisite means of locomotion and of subsistence.

1. Strategy regards the theatre of war, rather than the field of battle.  It selects the important points in this theatre, and the lines of communication by which they may be reached; it forms the plan and arranges the general operations of a campaign; but it leaves it to the engineers to overcome material obstacles and to erect new ones; it leaves to logistics the means of supporting armies and of moving them on the chosen lines; and to tactics, the particular dispositions for battle, when the armies have reached the destined points.  It is well to keep in mind these distinctions, which may be rendered still more obvious by a few illustrations.  The point where several lines of communications either intersect or meet, and the centre of an arc which is occupied by the enemy, are strategic points; but tactics would reject a position equally accessible on all sides, especially with its flanks exposed to attack.  Sempronius at Trebbia and Varro at Cannae, so placed their armies that the Carthagenians attacked them, at the same time, in front, on the flanks, and in rear; the Roman consuls were defeated:  but the central strategic position of Napoleon at Rivoli was eminently successful.  At the battle of Austerlitz the allies had projected a strategic movement to their left, in order to cut off Napoleon’s right from Vienna; Weyrother afterwards changed his plans, and executed a corresponding tactical movement.  By the former there had been some chance of success, but the latter exposed him to inevitable