To permit a retirement before such a great superiority of the enemy to be made without disaster, it was necessary to counter-attack not only at this inception of the movement, but throughout all the terrible strain of the ensuing eight days.
Here it may be necessary to explain why, in any retirement, continual counter-attacks on the pursuing enemy are necessary.
It is obvious that, under equal conditions, the pursuing enemy can advance as fast as can your own troops which are retreating before him. If, therefore, a retreat, once contact has been established, consisted in merely walking away from the enemy, that enemy would be able to maintain a ceaseless activity against one portion of your united force—its rear—which activity would be exercised against bodies on the march, and incapable of defence. To take but one example out of a hundred: his guns would be always unlimbering, shooting at you, then limbering up again to continue the pursuit; unlimbering again, shooting again—and so forth; while your guns would never reply, being occupied in an unbroken retirement, and therefore continually limbered up and useless behind their teams.
A retiring force, therefore, of whatever size—from a company to an army—can only safely effect its retirement by detaching one fraction from its total which shall hold up the pursuit for a time while the main body gets away.
When this detached fraction is wearied or imperilled, another fraction relieves it, taking up the same task in its turn; the first fraction, which had hitherto been checking the pursuit, falls back rapidly on to the main body, under cover of the new rearguard's fire as it turns to face the enemy. And the process is kept up, first one, then another portion of the whole force being devoted to it, until the retirement of the whole body has been successfully effected, and it is well ahead of its pursuers and secure.
Sketch 47.
For example: two White army corps, I., II., as in the annexed diagram, each of two divisions, 1, 2, and 3, 4, have to retire before a greatly superior Black force, abcde. They succeed in retiring by the action expressed in the following diagram. White corps No. I. first undertakes to hold up the enemy while No. II. makes off. No. I. detaches one division for the work (Division 2), and for a short time it checks the movement of a, b, and c, at least, of the enemy. Now d and e press on. But they cannot press on at any pace they choose, for an army must keep together, and the check to a, b, and c somewhat retards d and e. They advance, say, to the positions δε.
Sketch 48.