The machine guns allotted to the flanks will find positions in the way already described. These positions must be chosen along the line of retreat so as to hold all ground from which fire can be brought to bear on the retreating columns, and from which the enemy can be successfully repulsed when he attempts to break through. If there has been time to form infantry flank guards, the machine guns will assist in strengthening their resistance, and in covering their withdrawal. When acting alone they should open fire on suitable targets at the longest ranges, but when supporting infantry they may be able to reserve fire and surprise the enemy from close range. Guns working together on a flank must keep in touch with each other by signal, and guns should work in sections in mutual support. A battery acting as a flank guard to a force retreating must be prepared to occupy a front of some two or three miles, and sections will often be in positions a mile apart in open ground. Directly the guns have taken up positions the battery commander should go off with scouts from each gun and select fresh positions along the line of retreat. He will give the signal for retirement, and if in action at the time one gun of each section should move off first under cover of the fire of the others; or where the situation permits, a flank section may be ordered to move first. It must sometimes occur that the cavalry on the flanks will be compelled to retire by a superior force, and when this is the case machine guns in position may be able to inflict considerable loss, should the pursuing cavalry follow them too closely while retiring. As soon as it is known that the main body has reached the rallying position and is ready to hold it, the machine guns can be withdrawn. This will not usually be possible in daylight, and as a rule it will take place after dark. The dispositions of the machine guns in the rallying positions will be the same as in the case with infantry in the defence. REAR GUARDS The prime necessity for the machine guns with a rear guard of a retreating force is mobility, and without this they will be of little service and become a hindrance rather than a help. “A rear guard carries out its mission best by compelling the enemy’s troops to halt and deploy for attack as frequently and at as great a distance as possible. It can usually effect this by taking up a succession of defensive positions which the enemy must attack in turn. When the enemy’s dispositions are nearly complete, the rear guard moves off by successive retirements, each party as it falls back covering the retirement of the next by its fire. This action is repeated on the next favourable ground.... A rear guard may also effectually check an enemy by attacking his advanced troops as they emerge from a defile or difficult country.”[32]