321. The major locates such fire, communicating, and cover trenches and obstacles as are to be constructed. He assigns companies to construct them and details the troops to occupy them. 322. The major reenforces the firing line in accordance with the principles applicable to, and explained in connection with, the attack, maintaining no more rifles in the firing line than are necessary to prevent the enemy's advance. 323. The supply of ammunition being usually ample, fire is opened as soon as it is possible to break up the enemy's formation, stop his advance, or inflict material loss, but this rule must be modified to suit the ammunition supply. 324. The major causes the firing line and support to fix bayonets when an assault by the enemy is imminent. Captains direct this to be done if they are not in communication with the major and the measure is deemed advisable. Fire alone will not stop a determined, skillfully conducted attack. The defender must have equal tenacity; if he can stay in his trench or position and cross bayonets, he will at least have neutralized the hostile first line, and the combat will be decided by reserves. 325. If ordered or compelled to withdraw under hostile infantry fire or in the presence of hostile infantry, the support will be posted so as to cover the retirement of the firing line. 326. When the battalion is operating alone, the support must be strong and must be fed sparingly into the firing line, especially if a counterattack is planned. Opportunities for counterattack should be sought at all times. THE REGIMENT. [Illustration: Plate IV. THE REGIMENT.] 327. Normally, the regiment consists of three battalions, but these regulations are applicable to a regiment of two or more battalions. Special units, such as band, machine-gun company, and mounted scouts, have special formations for their own use. Movements herein prescribed are for the battalions; special units conform thereto unless otherwise prescribed or directed. 328. The colonel is responsible for the theoretical instruction and practical training of the regiment as a whole. Under his immediate supervision the training of the units of the regiment is conducted by their respective commanders. 329. The colonel either gives his commands or orders orally, by bugle, or by signal, or communicates them by staff officers or orderlies. Each major gives the appropriate commands or orders, and, in close-order movements, causes his battalion to execute the necessary movements at his command of execution. Each major ordinarily moves his battalion from one formation to another, in column of squads, in the most convenient manner, and, in the presence of the enemy, in the most direct manner consistent with cover.