Every commander will endeavour to prevent interference with his plans and future movements, and while striving to surprise and outwit the enemy he will exert every endeavour to prevent the application of this vital principle by the enemy. The commander of a force that is at rest will require security for that force in order that its rest may be undisturbed, and he will require the security to be assured in order that his plans for the overthrow of the enemy may be developed. He will, therefore, detach a portion of his force to ensure this security by observation, to prevent the secret occupation of localities the hostile possession of which will interfere with his plans; and by resistance to hostile movements he will secure the rest of the Main Body.

The force detailed to protect troops at rest is known as Outposts, and their duty is to preserve the security of the Main Body. Outposts protect the Main Body from surprise by Observation, and if attacked they gain time by Resistance until the commander of the Main Body can put his plans into execution by the occupation of the position in which he intends to receive attack. Observation is carried out by Aircraft, by Patrols (mobile troops by day and infantry by night), and by Sentries; Resistance is provided by Sentry Groups and by troops {130} in defensive positions, called the Piquets, which have other troops as Supports. In certain cases a Local Reserve and a General Reserve are also provided.

STRENGTH.—Work in the Outpost Line is most exhausting. Not a man or a horse should be employed there if their services can be dispensed with, and although the number of troops allotted for the work depends almost entirely upon the nature of the country and the tactical situation, it is laid down in the text-books that if an unnecessarily large proportion of the whole force is so employed the force will suffer in efficiency. It can also be seen that although the work is of the first importance and fraught with the greatest difficulties, it is clearly possible for a comparatively small body of troops to carry it out. Observation requires intelligence and vigilance rather than numbers; Resistance can be provided by the Delaying Action on a wide front of small numbers of skilled troops with the relative advantage conferred upon them in defence by machine guns and small arms, and with the assurance of support from their Main Body close at hand.

OBSERVATION.—A force can only be regarded as secure from surprise when every body of the enemy within striking distance is so closely watched that it can make no movement by night or day without its becoming known immediately to the observers of the Outposts. By day the Outpost commander will carry out Reconnaissance some distance ahead of his position by means of Aircraft and Patrols of mounted troops and cyclists, while the commander of each Outpost company keeps the approaches to the position under observation by sentries, so posted as to see and hear unobserved by a hostile force. By night, the Aircraft and mounted troops are unable to render much assistance as moving patrols, and the work of Reconnaissance and Observation falls upon the platoons of the Outpost companies.

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RESISTANCE.—For the purposes of Resistance the Outpost commander will rely upon his infantry and upon such artillery and machine guns as may be allotted to him, and if the area he is occupying is that in which the commander of the Main Body will meet attack the Outposts will be provided with a greater proportion of artillery and machine guns. Resistance is offered by the entrenchment of each Sentry Group in an all-round post, and depth and elasticity are given to the defence by the establishment of entrenched Piquets in selected, mutually supporting positions commanding with their fire every avenue of approach, covering the flanks of neighbouring Piquets, and so arranged in plan as to bring converging fire upon the enemy as he advances to the attack. These Piquet positions will be strengthened, when required, by the Supports, who will either assist in manning the defences of the Piquets or will occupy similarly prepared defensive posts on the flank. Local Reserves may sometimes be required for local counter-attacks, and in certain cases a General Reserve is provided. The degree of Resistance to be offered by the Sentry Groups depends on the tactical situation and will be specified by the Outpost commander. In certain cases the Sentry Groups are permitted in face of a heavy attack to fall back to the Piquets, but if they do so they must be warned of the danger of arriving headlong on the Piquet only just ahead of the enemy. In consequence of this danger such retirements are rarely permissible at night. The Piquets are generally posted on the Outpost Line of Resistance, in which case they hold their positions to the last man and the last round, until further orders are received from the commander of the force protected.

DISTANCE.—The distance of the Outpost position from the troops protected is regulated by the time the latter will require to prepare for action and by the importance of preventing the enemy's field artillery from {132} approaching within effective range of the ground on which these troops will deploy if attacked. Heavy guns and mortars, although motor traction gives them great mobility, are unlikely to accompany the enemy's Advanced Guard, and preparation to withstand or prevent their fire will not usually be required from Outpost troops. The effective range of shrapnel is 5,500 yards, the limit of the effective range of machine guns is 2,000 yards, and of Lewis guns and rifles the effective limit is 1,400 yards. The position on which the Main Body will deploy will thus be protected from the shrapnel of field artillery, if the possible fire-positions of that arm are brought under effective fire from machine guns 3,500 yards from the Position of Deployment, with Lewis guns and rifles about 500 yards further forward. On the other hand, especially in the case of small forces (against which artillery will not be likely to be sent), the distance must not be such as would permit of the Outposts being cut off, or as would necessitate the employment of an undue proportion of men on Outpost duty.

THE OUTPOST COMMANDER.—Before halting, a commander should first decide on his dispositions in case of attack, and then arrange the quartering of his command and the general position of the Outposts. In the case of a small independent force the commander of the force will usually himself detail the whole of the Outpost troops, and will either retain the command in his own hand or appoint an officer to command them, In such a case the disposition of the troops will probably be that of a perimeter camp, preparation being made against attack from all directions. In the case of large bodies Outpost troops will usually consist of all arms, and a definite commander will always be appointed. This commander will, when necessary, divide the Outpost line into sectors, delegating responsibility for the holding of each sector to the commander of a subordinate unit or formation, and defining the limits {133} of sectors by distinctive features such as trees, cottages, or streams. The tops of hills or the bottoms of valleys are not suitable as tactical boundaries, and roads should be inclusive to one or other sector, for a road used as a boundary may be neglected by one of the commands it divides under the impression that it is the duty of the other command to patrol it.

INFORMATION AND ORDERS.—The Outpost commander must have definite information on the following points:—

I. What is known of the enemy and information concerning friendly bodies of troops working against the enemy.