Beginning at the front we have the line of wire entanglements or obstacles with their listening posts X, for guarding them. Connecting the listening posts to the fire trenches are the communicating trenches. The fire trenches are shown by the heavy black line running about 60 feet in rear of the obstacles. Note the many traverses shown by the indentations in the line. Points marked M with arrows projecting to the flanks are machine guns, so located as to sweep the front of the position with a cross fire. Points marked S are underground shelters for from 3 to 6 men. Points marked S' are shelters for 30 men. In rear of the firing trenches at a distance varying from 100 to 200 feet is the line of cover trenches. This line is connected with the fire trenches by the zig zagged line of communicating trenches. Note that the latrines (L) and first aid stations (F) are just off from the communicating trenches, while the larger shelter for men (S') are near the cover trenches. As the note on the diagram shows, the trench requires 250 men to occupy it with double that number in support. The trench has 108 loopholes with spaces between provided with a higher banquette so that the whole parapet may be manned for firing.
On the battlefields of Europe today there are generally three lines of fire trenches. This permits the defender to fall back to a 2nd or 3rd prepared position in case he is driven out of his first trench. On a hill we find a fire trench near the foot of the slope, one just forward of the military crest, and the third on the reverse slope of the hill.
In many instances the first line trenches consist of as many as four or five lines of trenches running in a general lateral direction and connected by deep narrow communicating trenches. The depth between the first and last of these trenches is, in some instances, not over a hundred yards. Sign boards are necessary at short intervals to prevent the soldiers from getting lost. The effect of having so many alternative firing trenches is to make it extremely difficult for an enemy to advance from, or even to hold one of them, even when he gains a footing, as he would be swept by fire from the supporting trenches in rear and also by flanking fire from the adjacent trenches.
[1161]. Location. There are two things to be considered in locating trenches: (1) The tactical situation, and (2) the nature of the ground. The first consideration requires that the trenches be so located as to give the best field of fire. Locating near the base of hills possesses the advantage of horizontal fire, but, as a rule, it is difficult to support trenches so located and to retreat therefrom in case of necessity. While location near the crest of hills—on the "military crest"—does not possess the advantage of horizontal fire, it is easier to support trenches so located and to retreat therefrom. Depending upon circumstances, there are times when it will be better to intrench near the base of hills and there are other times when it will be better to intrench on the "military crest," which is always in front of the natural crest. The construction of trenches along the "military crest" does not give any "dead space"—that is, any space to the front that can not be reached by the fire of the men in the trenches.
Whether we should construct our trenches on high or low ground is a matter that should always be carefully considered under the particular conditions that happen to exist at that particular time, and the matter may be summarized as follows:
The advantages of the high ground are:—
1. We can generally see better what is going on to our front and flanks; and the men have a feeling of security that they do not enjoy on low ground.
2. We can usually reënforce the firing line better and the dead and wounded can be removed more easily.
3. The line of retreat is better.