The rear guard is charged with the important duty of covering the retreat. When a commander decides to retreat, he issues the necessary order. During a retreat the outpost for the night usually forms the rear guard of the following day. STRENGTH AND COMPOSITION The strength of a rear guard depends upon the nature of the country and the strength and character of the pursuing force. It can not, like the advance guard, count on the support of the main body. Machine guns are especially useful in the passage of defiles and in covering the crossings of rivers. Engineers and ambulance companies are usually assigned to rear guards. The troops of a rear guard are selected from those that have had previous local successes, or have suffered little loss and are comparatively fresh. DISTRIBUTION OF TROOPS The proximity and conduct of the enemy control, to a large extent, the formation of a rear guard. When it is not necessary to withdraw in deployed lines, the greater part of the rear guard marches on the road in column of route, taking up a formation resembling that of an advanced guard faced to the rear. The distribution of troops is therefore similar to that of an advance guard, namely: Reserve. Support. Rear cavalry. The rear cavalry is that portion of the rear-guard cavalry following the support. The support, as in an advance guard, is divided into two parts; that part nearest the enemy is called the rear party and marches with a rear point. DISTANCES The distance of the rear guard from the main body and between the fractions of the rear guard are about the same as in the case of an advance guard. If marching at night, the rear guard draws nearer the main body. REAR GUARD OF AN ADVANCING FORCE If there is a possibility that the rear of the column may be attacked, a rear guard of suitable strength and composition is provided, its conduct is practically the same as that of the rear guard of a retreating force. It generally marches in rear of the trains, those organizations following the combatant troops without distance. OUTPOSTS The size and disposition of the outpost will depend upon many circumstances, such as the size of the whole command, the proximity of the enemy and the situation with respect to him, the nature of the terrain, etc. A suitable strength may vary from a very small fraction to one third of the whole force. For a single company in bivouac a few sentinels and patrols will suffice; for a large command a more elaborate outpost system must be provided. It should be no stronger than is consistent with reasonable security. The most economical protection is furnished by keeping close contact with the enemy by means of outpost patrols, in conjunction with resisting detachments on the avenues of approach. The outpost should be composed of complete organizations.