On the march these detachments facilitate the advance of the main body by promptly driving off small bodies of the enemy who seek to harass or delay it; by removing obstacles from the line of advance; by repairing roads, bridges, etc., thus enabling the main body to advance uninterruptedly in convenient marching formations.
They protect the main body by preventing the enemy from firing into it when in close formation; by holding the enemy and enabling the main body to deploy before coming under effective fire; by preventing its size and condition being observed by the enemy; and, in retreat, by gaining time for it to make its escape or to reorganize its forces.
As the principal duty of these bodies is the same, viz., that of protecting the main body, there is a general similarity in the formations assumed by them. There is (1) the cavalry covering the front; next, (2) a group, or line of groups, in observation; then (3) the support, or line of supports, whose duty is to furnish the observation groups, and check the enemy pending the arrival of reinforcements; still farther in rear is (4) the reserve.
An advance or flank guard commander marches well to the front, and, from time to time, orders such additional reconnaissance or makes such changes in his dispositions as the circumstances of the case demand.
In large commands troops from all arms are generally detailed, the proportion from each being determined by the tactical situation; but commanders detail no more troops than the situation actually requires, as an excessive amount of such duty rapidly impairs the efficiency of a command. As a general rule troops detailed on the service of security vary in strength from one twentieth to one third of the entire command, but seldom exceed the latter. When practicable, the integrity of tactical units is preserved.
In mixed commands infantry usually forms the greater part of the troops detailed to the service of security. Cavalry is assigned to that duty whenever advantage can be taken of its superior mobility. The kind and amount of artillery are determined by circumstances.
The field trains of troops on this duty generally remain with the field train of the command, but if conditions permit they may join their organizations.
Troops on the service of security pay no compliments; individuals salute when they address, or are addressed by, a superior officer.
ADVANCE GUARDS
An advance guard is a detachment of the main body which precedes and covers it on the march.