So the assault was to be delivered by tanks, supported by relatively small detachments of infantry, while the R.A. F. were ordered to co-operate to their utmost capacity. Every available machine fit for offensive work was to be employed in the operations, the idea being not only to paralyse the Huns in the firing-line, but to prevent reinforcements and supplies reaching them. In brief, the whole of a certain German sector was to be wiped out. At five in the morning, or two hours before dawn, the tanks were to start upon their grim errand. Every square foot of ground occupied by the enemy in the coveted sector had been photographed and re-photographed by daring airmen. The work had been efficiently performed, but at a cost, as the long R.A.F. casualty list testified. It was not in the heat of combat that these daring aerial photographers had been shot down, but in the cold, methodical pursuit of an art that the demands of modern warfare had relentlessly absorbed. With an accurate knowledge of the nature of the terrain the task of the tanks had been rendered fairly straightforward. There were, of course, hidden pitfalls which the almost all-seeing lens of the camera failed to detect: cleverly-camouflaged gun-emplacements and nests of machine-guns that were not shown on the finished photograph-prints; but even here the work of the airman was evident. Cryptic markings on the prints gave the staff officers certain clues--an anti-aircraft battery here; a booby-trap there, an observation-post in that place. The science of detecting screened pitfalls was almost as perfect as the skilful art of camouflage. There were tanks and tanks. The ground trembled under the pulsations of their powerful engines. Whippets, male tanks, female tanks, "Rolls" tanks capable of doing twenty miles an hour with their 250-h.p. engines; tanks mounting six hundred quick-firers, tanks bristling with machine-guns--a veritable armada of land-ships moving forward in what appeared to be a solid, compact mass. They moved slowly at first, each section led by an officer on foot towards the as yet invisible German lines. There had been a spell of quietude on this part of the front of late. The Huns considered their defensive works so perfect that a frontal attack would be impossible, and, being let severely alone, they had refrained from their usual lavish display of star-shells. Grunting, groaning, coughing; ejecting vile, sulphurous fumes from their noisy exhausts, the steel-clad mastodons ambled onwards until Fritz, suddenly aware that danger was at hand, opened a furious fire that threw a dancing, lurid glare upon the crater-pitted plain over which the hordes of tanks surged like a sullen ground-swell beating upon a flat shore. Vivid red and white rockets--Fritz's S.O.S. signals--soared skywards, an appeal by the field-grey infantry for support from their heavy artillery.