y 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.

Its duties are:

(1) To guard against surprise and furnish information by reconnoitering
to the front and flanks.

(2) To push back small parties of the enemy and prevent their observing,
firing upon, or delaying the main body.

(3) To check the enemy's advance in force long enough to permit the main
body to prepare for action.

(4) When the enemy is