Well, in the first place the withdrawal of the British forces from the Aisne had to be carried out in detail. To have effected the withdrawal in mass would at once have aroused the observation and suspicion of the enemy. Next the forces thus withdrawn in detail, and in detail replaced by French troops, had to be massed at a convenient place secure from hostile intelligence hunters. Finally this main body of the British army had to be sent forward to the new line of front as a whole. Thus it was that the 2nd Army Corps, under the command of General Sir H. Smith-Dorrien, detrained at Bethune on the same day, October 11, that the 3rd Army Corps under the command of General Pulteney detrained at St. Omer. These towns are some twenty miles apart. Coincidently with the detrainment of the infantry and the guns, the 2nd and 3rd Divisions of the British cavalry advanced under the command of General Allenby and occupied the little town of Aire, which lies nearly half-way between them. By this move a front was formed from near Lens, where the French line ended across the country north-westward to the coast. The gap, so far as outflanking the Allied forces was concerned, was closed. In the latter part of September, as a prelude to their scheme, the Germans had occupied Lille. An occupying force which they had left there on their advance towards Paris had been driven out by the British. They now detached for the seizure of the city the 19th Active Corps and the 7th Reserve Corps. In the face of this overwhelming strength the British troops in Lille, part only of a division from Dunkirk, had no alternative but to retire. It was a bitter day for the inhabitants of Lille which witnessed the departure of these defenders, welcomed only a little while before with every demonstration of public joy. Besides these two German army corps who, based on Lille, began at once to drive westward towards Boulogne, a powerful mobile column, consisting of four divisions of cavalry, supported by horse artillery and three brigades of Jaegers, crossed the Lys, and passing to the south of Ypres, made a dash through Bailleul for Hazebrouck, covering at once the flank of the main advance from a possible attack from Dunkirk, and carrying out a turning movement against such French forces as were then supposed to be holding Bethune. These German troops, the two army corps and the flying column, though mustering in all more than 150,000 men, were only the vanguard of the mass intended to be thrown forward. It is clear that their expectation was that of attack from Dunkirk on the one side and from Bethune on the other. The flying column advanced to Hazebrouck, and the cavalry occupied the Forest of Nieppe to the south of that town without opposition.