It may be possible to have a line of protection extending around the entire camp. It must be extended and arranged so as to keep the enemy so far away from our main body that he cannot observe our numbers or our position. The enemy must not be permitted to approach close enough to the main body to annoy or surprise it. Experience shows that all of this is best accomplished by placing: 1st, some groups or line of groups farthest from our main body and closest to the enemy in order to observe, to report the movements of the enemy, and, when necessary, to make a temporary resistance; 2d, a line of resistance ("supporting groups") called "supports" upon which the first line can retire before, being swamped by superior numbers; 3d, large groups, or line of groups ("line of reserves"), so located that they may go to the assistance of the second line in case of necessity. Such arrangements may be illustrated by the following diagram.
Note that distances from the line of observation to the main body increase as the groups increase in size. The reserves are the largest groups. The groups on the Line of observation are the smallest.
It is most important to note that the groups are placed according to the conditions and circumstances of the particular case. Don't follow any blind rules. Your judgment must tell you when to place this group here and not to place that group there. Have as few men on such duty as practicable.
If a swamp, or a large body of water here, very small groups will afford the necessary security.
If a forest, or steep hills here, very small parties will afford the necessary security.
Assume that we want to afford security for our main body from any especially dangerous sector such as ABC. Our cavalry is in front of our first line and in touch with the enemy. The danger zone represents the direction from which the enemy is expected.]
This plan must be modified according to the particular case. Let us suppose that we are camping by a large body of water, or that we are surrounded by mountains. We can easily imagine where we could change the above general plan so as to give adequate protection and at the same time lessen the number of men detailed for security. We must never forget that men are generally tired when they arrive in camp, and that we should make their work as light as circumstances permit. It requires a nice judgment to choose the correct number for security.
We should know the names of these groups. Farthest away is the line that sees, and reports what it sees, but can offer only a limited resistance. This is called the "line of observation" or the "line of outguards." In rear of the line of outguards we have larger groups placed at greater distances. These are called "supports." This is the line that fights. This is the line that makes extensive preparations for fighting (or resisting). It is called the "line of supports" or the "line of resistance."[2] We have one farther and last line of groups which is still larger and occupies still greater distances than the two we have just discussed. This is the safety valve and is called the "reserve," or the "line of reserves." This is the line that gives a sound factor of safety. It will only be called upon in cases of emergency and may therefore generally enjoy a considerable degree of repose. But it and the line of supports combined must have sufficient strength to delay the enemy, in case of a general attack, long enough for our main body to form for battle.
Let us look at the line of outguards for further important considerations and distinctions. The enemy's movements and operations should ordinarily be expected where there are for him least difficulties. Large (dangerous) bodies of troops find trouble in marshes, thick forests, steep mountainous country. They avoid these obstacles as much as possible, selecting open country, solid soil, strong bridges, and good roads. Here is where large and strong groups in opposition are necessary. Small and unimportant groups (or no groups at all) should be placed where the enemy's advance is exceptionally difficult. Finally, there will be places between these last two extremes that require just an average amount of attention, that is to say, require groups of medium strength.