During the Battle of the Somme a new weapon had appeared on the Allied side. This was the Tank (so called because some unrevealing name had to be found for a device developed in secret). It was a machine shaped like a monstrous toad, which mounted machine-guns and light artillery, and could force its way through wire and parapets and walls, and go anywhere except in deep mud. Its main tactical use was to break down wire entanglements and to clear out redoubts and nests of machine-guns. When first used at the Somme the Tanks won a modified success, and in the following spring at Arras they fully justified themselves. Presently they began to develop into two types, one remaining heavy and slow and the other becoming a "whippet," a type which was easy to handle and attained a fair speed. Ultimately, as we shall see, they were to become the chief Allied weapon in breaking the enemy front, and also to perform the historic task of cavalry and go through the gaps which the infantry had made. In September 1917, while two British Armies were fighting desperately in the Ypres Salient for the Passchendaele Ridge, Sir Julian Byng’s Third Army, on the chalky plateau of Picardy, was almost idle. An observer might have noticed that General Hugh Elles, the commander of the Tank Corps, was a frequent visitor to Sir Julian’s headquarters at Albert. The same observer might have detected a curious self-consciousness during the following weeks at Tanks headquarters. Tanks officers, disguised in non-committal steel helmets and waterproofs, frequented the forward areas of the Third Army. Tanks motor-cars seemed suddenly to shed all distinguishing badges, and their drivers told lengthy and mendacious tales about their doings. Staff officers of the Tanks were never seen at any headquarters, but constantly in front-line trenches, where, when questioned, they found some difficulty in explaining their business. At the headquarters of one Tanks brigade there was a locked room, with "No Admittance" over the door, and inside—for the eye of the possible enemy spy—a quantity of carefully marked bogus maps. Some mystery was being hatched, but, though many hundreds suspected it, only a few knew the truth. On the 20th October it had been decided to make a surprise attack towards Cambrai, and to prepare the way for the infantry by Tanks instead of guns. The Third Battle of Ypres had brought the reputation of these machines very low. They had been used in the bottomless mud of the Salient, where they had no chance of being successful, and the generals in command had reported adversely on their merits. It was argued that they could not negotiate bad ground, that the ground on a battlefield must always be bad, and that, consequently, they were of no use on the battlefield. The first statement was doubtful, and the second false; but certainly if all battles had been like the Third Battle of Ypres the conclusion would have been justified.