The first requisite for a unit commander is a knowledge of the fundamental principles of the tactics of his arm and its employment in combination with the other arms, especially with the artillery. He must have a clear conception of the modern battlefield in order to understand for what he must train his unit. He must train it for battle conditions, not peace conditions. A company trained to be handled exclusively by word of command, as in a close order drill on the parade ground, lacks the training that fits for battle. [Sidenote: Needs of the unit commander] [Sidenote: Essential and desirable instruction] The more the enlisted men know of the art of war the better. Time is not available however, to teach them the whole art of war even if the men in ranks were capable of mastering it. The instructor must therefore clearly understand what are the things the men _must_ know and what are merely _desirable_ as additions to their training. Every effort should be spent and all available time devoted to first perfecting the men in the things they _must_ know; afterwards, if more time is available, it is well to extend in other directions their education and training. [Sidenote: Close order drill] Under the first heading, _essentials_, the men must be taught their close order drill. This is necessary for two reasons: it renders possible the orderly movement of troops and it makes for discipline. So far as the orderly movement of troops is concerned very little is necessary but without precision close order drill is of no value towards discipline. To have a disciplinary value, drill movements must be carried out with exactitude. Discipline is injured if, when an officer gives a command at drill, it is only carried out approximately; the soldier is acquiring the habit of slighting his work and of doing an approximation, not the precise duty demanded of him. It takes but little longer to learn to execute correctly the few movements prescribed than to learn them incorrectly, but it requires constant attention on the part of the instructor to maintain exactness. The attention and effort required on the part of the instructor are, however, amply compensated by the results. [Sidenote: Extended order drill] The men must know perfectly the mechanism of the extended order drill. A company must be able without confusion or mix-up to form line of skirmishers in the least possible time from any formation and facing in any direction. This will necessitate much practice. It does not take long to learn to form line of skirmishers quietly, from line or column of squads, facing to the front; but that is not sufficient. [Sidenote: Other essentials] The men must know how to estimate distances, how to shoot, how to use the bayonet; they must understand patrolling and outpost duty, the construction of hasty intrenchments, the application of first aid, how to cook the ration and how to care for their arms and equipment.