Whilst the German line was falling back to its new positions, and the Allies were eagerly following it across the ravished countryside until the increased resistance and the familiar lines of barbed wire warned them that the immediate retreat had come to an end, Sir Douglas Haig had managed, without relaxing his pursuit, to collect a strong striking force at the point of junction between the new German line and the old. The blow which he contemplated was no small local advance, but was a wide movement extending from the neighbourhood of Lens in the north to Arras in the south, a front of more than twelve miles. Upon this sector a tremendous concentration of artillery had been effected, and four corps were waiting the signal for the assault, the three southern ones forming Allenby's Third Army, while the fourth or northern one was the right-hand corps of Horne's {21} First Army. The southern corps were the Seventh (Snow), which operated to the south of Arras, having Croisilles for its southern boundary; the Sixth (Haldane), which advanced due east from Arras with the Scarpe for its northern boundary; the Seventeenth (Fergusson), which had its right on the Scarpe and its left on Thelus, with its front facing the three spurs which form this end of the Vimy Ridge; and finally the Canadian Corps (Byng), which faced this long and sinister slope, the scene of so much bloodshed in the past. Each corps was marshalled with three divisions in front and one in reserve, so that there were roughly 120,000 men in the storming line with 40,000 advancing behind them. Maxse's Eighteenth Corps was in reserve in the rear of the Third Army, while M'Cracken's Thirteenth Corps was behind the First Army. The Germans had six divisions, the Eleventh Prussian, Fourteenth Bavarian, First Bavarian Reserve, and the Seventeenth, Eighteenth, and Seventy-ninth reserve in the line. Their guns also were numerous, as subsequent captures were to prove, but it is probable that an extension of the Hindenburg retreat was in contemplation, and that some of the heavy artillery was already on the move. A second strong line from Drocourt to Queant was known to exist, and its occupation would form a natural sequel to the retirement in the south.