DISTANCE.—The distance at which the Rear Guard works is governed by the duty it has to perform, viz.: to permit the withdrawal of the Main Body to be carried out without interruption by the enemy, and to effect this it will usually be necessary for the Machine Gun and Infantry Platoons of the Main Guard to keep within effective range of positions from which hostile artillery might molest the Main Body. The commander will probably remain with this part of his force, as its work is of the highest importance; in any case his position must be made known and there should be definite commanders of the Rear Party and the Main Guard. But while the distance separating the Rear Guard from the Main Body must be sufficient, it must not be too great, or the enemy may penetrate between it and the Main Body, and not only will the Rear Guard be cut off and liable to destruction but it will cease to protect the Main Body.
TACTICAL PRINCIPLES.—The tactical work of a Rear Guard is carried out according to the following principles:—
The Rear Party watches, and it must watch all the roads and tracks by which the pursuing force can advance, and is responsible that the enemy does not get round the flanks (which may or may not be specially protected by Flank Guards). Reconnaissance by Aircraft for the discovery of intended outflanking movements is probably of greater value in Rear Guard work than in any other military action. The Rear Party also resists the hostile advanced troops as long as possible, withdrawing before it is outflanked. "An outflanking manoeuvre is specially convenient when attacking a Rear Guard, for the latter cannot fulfil its mission once it has been turned" (Marshal Foch).
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The Main Guard fights for time. If the withdrawal is more or less unmolested, or if such pursuit as is offered can be dealt with by the Rear Party, the Main Guard can continue its march, taking care not to close in on the Main Body; and while falling back it can demolish bridges, create obstacles, prepare ambushes, and so on, employing all devices (within the laws of war) for delaying the enemy. When hotly pursued it must gain time at all costs for the army it is covering, and must not allow itself to be driven back on to the Main Body; or it will hamper that force and cease to protect it. Time can be gained by compelling the enemy to halt to reconnoitre a position, by making him deploy into attack formation, and by making him go out of his way in order to envelop a flank. But before an attack reaches a position in such strength as to ensure success, and before the enveloping force can achieve its object, sub-divisions of the Main Guard will withdraw in succession under covering fire from those still in the line, which also withdraw in their turn under covering fire from the sub-divisions in their new positions, to tactical points further back, from which again they cover the withdrawal of the forces which had protected their own movement.
Certain points must be noted about the positions chosen for these successive fire-fights, and the choice of the positions is so difficult that an experienced staff officer should be specially detailed for the work, Positions chosen must be in the enemy's way and the lines of withdrawal to them must not converge; they must be easy to defend and difficult to attack; the flanks must be secure from direct attack and effective enfilade fire, necessitating a wide detour (and consequent gain of time from the enemy) before they can be threatened; long-range artillery fire on the lines of approach should be possible in order to delay and break up the enemy's advance; and each position chosen for the next line of resistance should be unseen by the {123} pursuing enemy, and sufficiently far away from the line last occupied to induce him to resume his march formation. This will necessitate a repetition on the part of the enemy of all the stages of the attack—the discovery and the report on the position, the decision to attack, and the deployment into attack formation. It will often be of advantage for a Rear Guard to take up a delaying position one or two hours before dark, as the enemy will then have to attack with darkness approaching and may wish to defer the attack until daylight, thus gaining several hours for the protected force.
"The first position taken up by a Rear Guard after an unsuccessful fight must be held longer, as a rule, than the subsequent positions, because when once the defeated army has got well away along the roads and has regained some semblance of organisation, the march continues without interruption unless some obstacle has to be crossed" (General Haking, "Staff Rides"). It can also be noted that as it is seldom the intention of the Rear Guard commander to deliver a decisive counter-attack, he can detail a very large proportion of his force to hold the successive positions, with local reserves, for purely local counter-attacks; and for the same reason, an obstacle in front of his position (which would make that position unsuitable for the Active Defence, as it would prevent the advance of the General Reserve to the decisive counter-attack) is most welcome in the Delaying Action of a Rear Guard fighting for time for its Main Body.
When at length a line of resistance is evacuated, the heavy artillery will be withdrawn first to move to a distant fire position, then the slow moving infantry and the light artillery (under the protective fire of the aircraft and mobile troops), and last the cavalry and other mobile troops, who by reason of their superior mobility, can hang on to the last and can protect the flanks of the Rear Guard as they fall back, before {124} resuming their work as a Rear Party, observing and resisting the advanced troops of the pursuing force.
During a close pursuit the Rear Guard commander will be called upon to exercise all his faculties and to exert all his tactical ability in handling his command. One of the most anxious times before him will be when the Main Body is passing through a defile, as such a passage will not only delay its march but will make its columns particularly vulnerable and helpless. In the case of defiles Napoleon's maxim must be borne in mind: "It is contrary to the principles of war to let one's parks and heavy artillery enter a defile if the other end is not held also." At Sannah's Post (March 31, 1900) the train was permitted to enter a defile caused by the banks of the Koorn River without the previous occupation of that defile, and all the wagons were captured. This not only emphasises the necessity for an Advanced Guard in retreat, but points to the need of tactical knowledge on the part of the Rear Guard commander, especially in mountainous country or in terrain cut up by woods and marshes, where the train is liable to cause delays, as the withdrawing force is compelled to march in a long drawn column. Extra time must be gained by the Main Guard to enable the Main Body to emerge from the defile. The Rear Guard commander must therefore adapt his plans to suit the country through which the Main Body has to pass, as well as the country in which he will himself fight Delaying Actions. A good map and ability to use it, and close co-operation with the Main Body, must be determining factors for success or failure.
TRAINING.—When troops are being exercised in Rear Guard work opportunities should be taken to explain the difficulties of choosing suitable positions, of withdrawing from them when involved in battle, of the paramount necessity for mutual support, and of accepting {125} any risk that may be required to safeguard the Main Body. Stress should be laid upon the importance of Fire Tactics (the judicious combination of Fire and Movement), the greatest of all factors in a successful Rear Guard battle, and upon the ability to read and understand a map, an essential qualification in all movements of troops and indispensable in Rear Guard fighting. From the map a platoon commander must be able to predict the probable line of the enemy's advance against the line of resistance as well as the best route to be taken when, at length, he withdraws his platoon to another fire position in rear; while he must be prepared to throw his platoon in local counter-attack on the flank or rear of an assaulting party that has become detached from its supports and therefore affords a fleeting opportunity for a local fighting success, and a rapid advance for this purpose along a route unseen to the foe, a speedy reorganisation after victory, and a rapid withdrawal to the point of issue, or to a line in rear, can best be achieved by use of the map and reconnaissance of the ground of the encounter.