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). {122} _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.