OBSTACLES
[1173]. Object. The main objects in placing obstacles in front of the trenches are, to protect them from surprise, and to stop the enemy's advance or to delay him while under the defender's fire.
[1174]. Necessity for obstacles. It is evident that the present tendency is to reduce the number of men assigned to constant occupancy of the first line trenches. This is due to the effectiveness of rifle fire at close range, the destructive effect of shell and shrapnel, the infrequency of daylight attack on intrenched positions, and the severe strain on the men. The aim seems to be the placing here and there of a lookout or trench guards, who, when necessity demands can call help from the near by splinterproofs, dugouts, etc., before the enemy can make his way through the obstacles. It has been found from experience in the European war that as long as shells are directed at the trenches no danger of attack is feared but, when the shells are concentrated against the obstacles the trenches are manned and preparations are made to resist an assault.
[1175]. Location. Obstacles must be so located that they will be exposed to the defenders' fire, and should be sheltered as far as possible from the enemy's artillery fire. They should be difficult to remove or destroy, should afford no cover for the enemy, and should not obstruct counter attacks. No obstacle should be more than 100 yards from the defender's trench. Care must be taken not to place them so close to the trench that hand grenades can be thrown into the trench from beyond the obstacle. Obstacles may be placed in one, two or three lines. As far as possible they should be concealed so that they will not betray the location of the trench.
[1176]. Kinds of Obstacles. The following are the most common kinds of obstacles:—
Abatis consisting of trees lying parallel to each other with the branches pointing in the general direction of approach and interlaced. All leaves and small twigs should be removed and the stiff ends of branches pointed.
Abatis on open ground is most conveniently made of branches about 15 feet long. The branches are staked or tied down and the butts anchored by covering them with earth. Barbed wire may be interlaced among the branches. Successive rows are placed, the branches of one extending over the trunks of the one in front, so as to make the abatis 5 feet high and as wide as desired. It is better to place the abatis in a natural depression or a ditch, for concealment and protection from fire. If exposed to artillery, an abatis must be protected either as above or else by raising a glacis in front of it. [Fig. 1] shows a typical form of abatis with a glacis in front. An abatis formed by felling trees toward the enemy, leaving the butt hanging to the stump, the branches prepared as before, is called a slashing, [Fig. 2]. It gives cover, and should be well flanked.
Plate I