The purpose of an obstacle is to obtain the control of the enemies in respect to direction and speed during an attack, and to deflect troops into areas favorable to their destruction by the defenders. They break up the unity of action, deflect parties isolated into the best swept fields of fire and hold them under close fire of the defenses. An obstacle should be close to the defender’s position, not more than 80 yards away at the most. A system is now in use on the Western Front where entanglements in front of trenches are placed approximately 20 yards from the parapet, as that has been found to be the distance at which the deadliest bombing can be done. They should be sheltered or screened, when possible, from enemy artillery, giving no cover to the enemy and be so placed as to surprise the enemy. They should not interfere with any counter attack necessary to be made and have occasional gaps, which may be mined. Types of obstacles are low wire entanglements, their height depending on the condition existing in No Man’s Land; high wire entanglements, barricades, mines, inundations, etc. Wire plays the important part in most of the obstacles now being used on all fronts, and is generally used, first, as a trip wire stretched just above the ground, or fastened in loose coils to short pickets. Flares and alarm guns and tin cans may be used in connection with this; second: a simple fence to cause delay and confusion to the enemy at night; third: as a concealed obstacle in fords and standing crops or long grass; fourth: as a help towards making hedges and brushwood impassable, and as a wire entanglement solely.

The wire entanglements are the best obstacles, as they are quickly and easily made, are very difficult to destroy and offer no obstruction to fire in view of the defenses.

Low Wire Entanglements. Stout sticks, 36 inches long and 1½ to 2 inches in diameter, are driven into the ground on level at 6-foot intervals. These should be driven in at least three rows so arranged that the sticks in one row are opposite the centers of gaps in the next. The heads of the sticks are connected by strong wires crossing diagonally from 12 to 18 inches above the ground.

High Wire Entanglements. To be effective, high wire entanglements should prevent the enemy from crawling through it at or near the ground level, and when possible, should be screened from enemy artillery observation. This obviously is utterly impossible as far as wire entanglements in front of the firing line is concerned, but it is possible to a certain extent to screen these high entanglements from observation in front of support lines and second line systems. Under conditions existing at the front, the wire work is often and generally, for very good reasons, of a hasty character, and it is best, therefore, to limit the first stage to just enough to form a nucleus of the whole entanglement, in order that the required area may be covered by obstacle before serious interruption occurs. To do this, the obstacle is best constructed in two zones, with a small space between. The pickets should be from 5 to 8 feet long and average five inches in diameter, being placed at irregular distances and with varying heights in order to make more difficult the passage over them by means of hurdles and planks. The outer pickets should be very firmly driven and stayed to prevent the enemy dragging the obstacle away. It is also nice to drive large nails into the tops of the posts with half their length projecting. After the posts are driven in, they are first joined diagonally, that is, from head to foot and foot to head, by winding the wire around each post and securing it by staples. Each set of posts should be stayed by four wires. There should be a trip wire 9 inches from the ground, or even less, running continuously round the outer posts, and another one foot from the top of the middle posts. The barbed wire can then be hung in festoons between the posts on no fixed pattern and fastened to them. It must also be fastened to the other wire where it crosses, by short lengths of wire especially cut beforehand. Slack wires are of more hindrance when cut than taut wires. Tight wires help the enemy’s advance by giving support to hurdles and other methods used to get across these entanglements. One method once used by the Germans was the carrying out of mattresses and endeavoring to cover the wire obstacles in that manner, and had the wire been taut it would have been a success, but as it happened, it was not.

The ground on the enemy’s side and within the entanglement, as a matter of courtesy, is strewn with broken glass and tangled wire. The whole system of entanglement should be under well-controlled machine gun fire from special points and should be widest where the fire of the defenders is least effective by night. There should be one sentry at least to each 50 yards of entanglement.

Abatis. A form of obstacle made by trees cut down and laid side by side as close as possible with their branches towards the enemy is used. These should be in a hollow and screened from view to make it of any value. The butts of the trees should be firmly secured by burying them in the earth, or by laying logs of timber across several butts. Wire and barbed wire must then be interlaced between the boughs, which should also be sharpened to points on the enemy side. Some of the lower branches may be pegged to the ground, if thought necessary, to insure the maximum resistance.

Barricades. These are used for the defense of streets, roads, bridges, etc., and are made of any available materials, including furniture and vehicles, either overthrown or with wheels removed, carts filled with earth, railings, bales of goods, etc. Where trees are growing along the roadside, fell them across it and entangle with wire.

It must be kept in mind, though, that passages are required through these barricades to allow outposts to advance or retreat, that these passages, viewed from the front, must not appear as openings. To avoid this the barricade should be made in two parts, one overlapping the other. Sometimes, where there is a gap in a row of houses, or a sharp bend in the road, a barricade may be made in one part and a passage round one end left for traffic.

Inundations. They may be formed by damming streams at convenient points, specially in the valleys, or by damming up the arches of bridges. In the latter case, care must be taken not to endanger the stability of the bridge. The ditches of field works form a good obstacle when flooded. Destroyed trenches in front of a breastworks may be filled with water, and with barbed wire thrown into it, will prove an effective obstacle.

Fougasses. These are used in connection with obstacles and are really land mines loaded with stones, bricks, etc. An excavation is made in conical shape with an axis inclined to about 40 degrees toward the enemy horizon. A box of powder is then placed in a recess at the bottom and on the box is placed a wooden platform or shield 3 to 4 inches thick, over which stones are piled.

A fuse is placed in a groove cut at the back of the excavation. A line of least resistance must be so arranged that by placing the excavated earth on the back edge of the fougasse, the powder will act in the direction of the axis and not vertically. A fougasse charged with 80 pounds of powder may be constructed in this manner to throw five tons of brick and stone over a surface about 160 yards long by 120 yards wide.

All of the foregoing are labors of working parties, as well as construction of dugouts, carrying of supplies, ammunition, etc., drainage and building of the trenches and the many other jobs behind the lines. Always, no matter how small the job, careful forethought must be given to the planning and arrangement necessary to carry it out.