Now that is exactly what you have in the case of a great line of trenches. Your defending force does not get weaker and weaker as it diminishes in number until it reaches zero; it is able to hold trenches of a certain length with a certain minimum of men, and when it falls below that minimum _it cannot hold the line at all_. It has to fall back upon a shorter line. Supposing you have, for instance, under such conditions as those of Diagram I, a line of trenches A-B holding the issue between two obstacles X and Y against an enemy who attacks from the direction E. The number of men holding these trenches, A-B, is nine units, and this number is just enough, and only just enough, to prevent an enemy attacking from E getting through. Nine units just prevent any part of the line of trenches, A-B, from being left defenceless. What does one mean by saying: “Just enough to prevent an enemy getting through?” [Illustration: DIAGRAM I. Suppose you have a line of trenches A-B holding the issue between two obstacles X and Y against an enemy who attacks from the direction E. The number of men holding those trenches is nine units, and this number is only just enough to prevent the attacking force getting through.] One means that if you consider trenches in detail, a certain length of trench needs a certain number of men to hold it, and if that number of men is not present, it must be altogether abandoned. It is evident that a mile of trench, for instance, could not be held by half-a-dozen men, even if the forces opposed to them were only a half-dozen. [Illustration: DIAGRAM II. Every man in a trench may be regarded as accounting for a certain angle of space in front of him, as A-B-C. If the extreme point at which you can stop a rush is the line L-L then you must have at least enough men--a-a-a--to cover that line with their fire.] You must, first, have enough men to cover the field of fire in front of the trench with the missiles from the weapons of each, and so stop the assault of the enemy. Every man with his rifle may be regarded as accounting for a certain angle of space in front of him as in the angles A B C and the other similar angles in Diagram II. These angles must meet and cover the whole ground, in theory at least, not further from the trench than the most advanced point to which it has been discovered that an enemy’s rush will reach before combined fire stops it. In practice, of course, you need very many more men, but the theory of the thing is that if the extreme point at which you can expect to stop a rush is the line L-L, and if the angle over which a rifle is usefully used is the angle B-A-C, then you cannot hold the trench at all unless you have at least enough men a-a-a just to cover that line L-L with their fire. If you try to do it with less men, as in Diagram III, you would only cover a portion of the front; you would leave a gap in it between X and Y through which the trench would be carried.