Where the tactical situation demands the holding of a line too long to be occupied throughout at this density, it is generally better to deploy companies or platoons at one man per yard, leaving gaps in the line between them, than to distribute the men uniformly at increased intervals.
[434.] A relatively thin firing line may be employed when merely covering the movements of other forces; when on the defensive against poor troops; when the final action to be taken has not yet been determined; and, in general, when fire superiority is not necessary.
[435.] The length of the firing line that the whole force may employ depends upon the density of the line and the strength in rear required by the situation.
Supports and reserves constitute the strength in rear.
In a decisive attack they should be at least strong enough to replace a heavy loss in the original firing line and to increase the charging line to a density of at least one and one-half men per yard and still have troops in rear for protection and for the other purposes mentioned above.
[436.] In the original deployment the strength of the reserve held out by each commander comprises from one-sixth to two-thirds of his unit, depending upon the nature of the service expected of the reserve.
A small force in a covering or delaying action requires very little strength in rear, while a large force fighting a decisive battle requires much. Therefore, depending upon circumstances, the original deployment, including the strength in rear, may vary from 1 to 10 men per yard. Against an enemy poorly disciplined and trained, or lacking in morale, a thinner deployment is permissible.
[437.] The density of the whole deployment increases with the size of the command, because the larger the command the greater the necessity for reserves. Thus, a battalion acting alone may attack with two men per yard of front, but a regiment, with three battalions, may only double the front of the one battalion.
[438.] By the assignment of divisions or larger units to parts of a line of battle several miles long, a series of semi-independent battle, or local combat, districts are created.
The general deployment for a long line of battle comprising several battle districts is not directly considered in these regulations. The deployments treated of herein are those of the infantry within such districts.