And every step beyond Cambrai, desolate as it is, is thronged with these invisible legions.  There to our right rises the long line of Bourlon Wood—­here are the sand-pits at its foot—­and there are the ruined fragments of Fontaine-notre-Dame.  There rushes over one again the exultation and the bitter recoil of those London days in November, 1917, when the news of the Cambrai battle came in; the glorious surprise of the tanks; the triumphant progress of Sir Julian Byng; the evening papers with their telegrams, and those tragic joy-bells that began to ring; and then the flowing back of the German wave; the British withdrawal from that high wood yonder which had cost so much to win, and from much else; the bewilderment and disappointment at home.  A tired Army, and an attack pushed too far?—­is that the summing up of the first battle of Cambrai?  A sudden gleam had shone on that dark autumn which had seen the bitter victory and the appalling losses of Passchendaele, and then the gleam vanished, and the winter closed in, and there was nothing for the British Army but to turn its steady mind to the Russian break-down and to the ever-growing certainty of a German attack, fiercer and more formidable than had ever yet broken on the Allies.

Bourlon Wood—­famous name!—­fades behind us.  A few rubbish heaps beside the road tell of former farms and factories.  The car descends a long slope, and then, suddenly, before us runs the great dry trough of the Canal du Nord; in front, a ruined bridge, with a temporary one beside it, a ruined lock on the left, and rising ground beyond.  We cross the bridge, mount a short way on the western slope, then in the darkening afternoon we walk along the front trench of the Hindenburg line, north and south of the road—­a superb trench, the finest I have yet seen, dug right down into the rock, with concrete headquarters, dressing and signal stations, machine-gun emplacements and observation posts; and, in front of it, great fields of wire, through which wide lanes have been flattened down.  Now we have turned eastward, and we stand and gaze towards Cambrai, over the road we have come.  The huge trench is before us, the waterless canal with its steep banks lies beyond, and on the further hill-side, trench beyond trench, as far as the eye can see, the lines still fairly clear, though in some places broken up and confused by bombardment.  The officer beside me draws my attention to some marks on the ground near me—­the track marks of two tanks as plain almost as when they were made.  One of them, after flattening a wide passage through the wire fields for the advance of the infantry, had clambered across the trench.  At our feet were the grooved marks of the descent, and we could follow them through the incredible rise on the further side; after which the protected monster—­of much lighter build, however, than his predecessors on the Somme—­seemed to have run north and south along the trench, silencing the deadly patter of the machine guns; while its fellow on the west side, according to its tracks at least, had also turned south, for the same purpose.