Early in 1917 it became known to our intelligence service that the enemy was contemplating retirement on a large scale from the Somme battle-front. Reports from prisoners and aeroplane photographs of a new line, famous afterwards as the Hindenburg line, running from west of Cambrai to St. Quentin, left in doubt only the date and manner of the withdrawal. To the latter question some answer was possible by reference to our mentors or from a text-book appreciation of the situation, though no one guessed until the movement had in reality started with what circumstances the Germans would see fit to invest it. The date was a more difficult problem. For its solution recourse must be had by commanders, staff officers and experts to the infantry. A competition open to all battalions holding the line (and without other entrance fee) thereupon commenced. To whom should fall the laurels of a correct diagnosis of the march-table of the German rear-guards, who be the first to scatter them by the relentless pursuit of our victorious arms? To our higher staff the question whether the enemy was still manning with normal garrisons the front opposite our armies seemed relatively simple. Readers, however, with experience of trench warfare will remember that in the line by day it was impossible to surmise correctly one item of what was happening a hundred yards away in hostile trenches; certainly one knew well enough when shells were falling, and 'minnies,' rifle-grenades and snipers' bullets argued that a pernicious, almost verminous, form of life was extant not far away: but despite all this, stared a sentry never so vigilantly, through his periscope he could hardly predict whether two, ten, or a hundred of the enemy tribe were hidden below earth almost within a stone's throw. At night it seemed probable that a patrol of a few brave men could crawl right up to the German wire and listen, or by setting foot in them enquire whether 'Fritz' was at home in his trenches or no; and so our patrols could, and did. In practice, however, our most active patrols were frequently deceived. Shots and Verey lights, which came from several directions, might be discharged by a solitary German, whose function it was to go the round of the enemy posts and fire from each spasmodically in turn. A trench entered and found empty might be a disused sap or bay habitually unoccupied. To maintain the normal semblance of trench-warfare was an easy task for the German, and one that he never failed in. Repeatedly in his retirements during the war he removed his real forces, his artillery and stores unbeknown to our watching infantry and their questioning staff. The screen of a retreating enemy is not easily caught up and pierced by an advanced guard not superior to it in strength and inferior in mobility. On the Somme in 1917 and from the Lys salient in 1918 the Germans retired from wide to narrower divisional fronts (giving themselves greater 'depth' in the process), which fact,