Fortifications.— Fortifications may be de fined as any engineering devices for increasing the fighting power of troops in the field. En trenchments, screens or blinds, obstacles, com municating trenches, mines and demolitions, all come under the designation of fortifications. Of these the most important are those which afford protection from the enemy's fire and incidentally provide concealment and means of intercommunication. The term fortification, as commonly employed, refers to works of this character only. Field works are usually con sidered as to (1) location, or siting; (2) trace, or ground plan; (3) construction; (4) con cealment. Engineers generally make three dis tinct classes of field or temporary fortifica tions: (1) all works devised for the temporary protection of important points, such as cities, arsenals, bridges, fords, positions, etc., tech nically known as field works; (2) the various devices of the engineer for reducing a forti fied place by means of parallels and approaches, known as siege works; (3) quickly made de fenses by which an army in the presence of an enemy protects itself, known as bottle entrench ments or hasty entrenchments.
The general line to be held or defended de pends upon strategical considerations and the location of trenches is affected by the general line to be occupied, tactical considerations and features of the terrain. Tactical considera tions, such as actual or potential interference by the enemy with the construction, may affect the location. All works for the defense of a position should provide concealment, a clear field of frontal fire, good communications to the rear, the flanks resting upon some natural obstacle or in contact with adjacent troops. A clear field of fire to the front was formerly considered all important and necessary to be secured at the expense of all other considera tions; but, with the greatly increased effective ness of modern artillery, it has been accepted as a maxim that, that which is seen is as good as destroyed, and concealment of the works be comes of first importance.
Organization of a The organ ization of a position does not imply the estab lishment of a continuous firing trench. The terrain in front of a position is best covered by a combination of frontal and flank fire from distinct elements, such as trenches, block houses, etc., separated by small intervals; thus economizing the personnel which is one of the advantages of fortification, and mini mizing the effect of the enemy's artillery fire. Each of these elements is occupied by a fixed garrison, which should always be a complete unit, the strength of which, variable according to circumstances, may be as much as a platoon. These elements are not uniformly distributed along the front; their distribution is deter mined by the features of the terrain, the neces sity of having a greater volume of fire at one point than at another, in one direction than another, and other tactical considerations. A
line can always be forced at some points so that it is necessary to have elements in rear which come into action when the first have fallen. It is, therefore, necessary to disperse the elements laterally and in depth. A group of elements, thus disposed, is known as a strong point, and by their mutual support should permit the garrison to hold and stop the enemy by its fire. It should be surrounded by a continuous obstacle and should always be oc cupied by a fixed unit responsible for its de fense. Strong points are grouped together lat erally and in depth to form supporting points and the combination of several supporting points under one commander forms a sector. This organization is a gradual development made during a prolonged occupation of a posi tion in trench warfare such as developed on the western front in Europe. In the initial occupation of a position the first step is the construction of the strong points for the firing line, with their firing trenches and obstacles first, then their cover, approach and communi cating trenches. The next step is the construc tion of the strong points for the support trenches, with the necessary communicating and approach trenches. Finally the defensive zone is completed by the construction of the reserve and intermediate trenches organized into strong points, and all of these into supporting points and sectors.
The object of an obstacle is to check a hostile rush and delay the enemy under the close fire of the defense, and it should be at such distance from the parapet that it is difficult for hostile bombers to crawl up to it and throw bombs into the trench. On the other hand, the obstacle should be under close observation and fire of the defense. If possible it should be concealed for purposes of surprise and to decrease its liability to injury by artillery fire; it should form no shelter for the enemy. Some form of wire entangle ment is ordinarily used. Obstacles are usually placed all around strong and supporting points and often on both sides of approach trenches. Passages through them for counter attacks are usually covered by machine gun fire. At im portant points the wire entanglement is placed in belts 20 feet wide, two or more belts about 20 yards apart being used. The modern form is a compromise between the high and the low wire entanglement and is generally from two to two and one-half feet high. The wire is strung loosely, and many forms of spirals and hoops have been devised. Barbed wire is ordi narily used; the posts are of wood or iron; the iron ones are provided with eyelets for attach ing the wire. There are usually three rows of posts set in quincunx order from six to eight feet apart.