Marshal Foch, after consultation with the British Commander-in-Chief, had desired the commanders of all armies, British and French, to prepare plans of action and to be ready to put them into operation at short notice. He now more particularly addressed inquiry to Sir Douglas Haig as to his willingness to undertake a continuous offensive towards the German centre. Sir Douglas Haig assented to Marshal Foch's representations as to the superior advantages to be gained from an attack there, and, while continuing a show of preparation in the Ypres area, where Ludendorff, on an estimate of the psychology of the British commander, would expect
the counter-attack to come, and was already taking steps to reduce its effectiveness by masked withdrawals, transferred forces steadily to Rawlinson's Fourth Army on the Somme. This army, and the First French Army under Debeney on the right, both directed by Haig, were set in motion on 8th Aug. on a 16-mile front from Morlancourt to Moreuil. The thrust was successful beyond expectation. The British Fourth Army, on the right wing, went through the German divisions of von Marwitz (Second Army) like paper, regaining the old outer-line defences of Amiens; and Debeney's men crushing the resistance of von Hutier's Eighteenth German Army, and reaching Fresnoy and Plessier, where they linked up with General Humbert's Third French Army on the road to Roye. The captures of the day amounted to 17,000 prisoners and 500 guns, an unmistakeable symptom that the German power of resistance was shaking.
On 9th Aug. Rawlinson pushed on still farther; on 10th Aug. General Humbert prolonged Debeney's still attacking line and took Montdidier, and a number of villages. These three armies continued to eat into the enemy's positions and to pin a number of German divisions down till 20th Aug., while Mangin's army at Soissons moved en échelon to take up contact with Humbert's right. Meanwhile Ludendorff, fully aware now that the initiative had passed out of his hands, and that the best course that lay open before him was a 'strategic retirement', began to effect one stage of it in the Ypres and Lys district under the direction of General Sixt von Armin, whose withdrawal was followed vigilantly by the British forces; and another stage in the German salient on the Ancre, where General von Below's Seventeenth Army was stationed. Von Below withdrew on the Bapaume line from Serre, Beaumont-Hamel, and Bucquoy to the shelter of the sector of the Hindenburg line behind it (13th, 14th, and 15th Aug.).
But whereas in 1917 Ludendorff had disconcerted both British and French Commanders-in-Chief by a sudden withdrawal on the Bapaume-Péronne line of the Somme, he was not now allowed to withdraw without injury. Haig's battle of Bapaume (21st Aug. and following days) was designed in two stages, the first of which brought up Byng's Third Army to a position in which it was aligned with Rawlinson's Fourth Army, and the second of which saw the Third and Fourth Army attack von Below in combination. The combined pressure of these two armies was continuously successful, though the Germans fell back stubbornly in many places. By 30th Aug. Bapaume was once again in British hands, and the line of attack was threatening the strongholds of the Hindenburg line, while its extension ran through Heudecourt and Fremiecourt to Cléry. Péronne fell to the Australian Corps by a most gallant feat of arms on 1st Sept. A more strategically significant victory was gained on the same day when the capture of Bullecourt, followed by that of Riencourt and Cagnicourt, opened up the first crevice in the ramifications of the Hindenburg defences known as the Drocourt-Quéant switch line. The battle of Bapaume drove thirty-five German divisions from the old Somme battlefield, and captured 34,000 men and 27 guns.
The crevice in the Drocourt-Quéant defences was still further widened on 1st Sept., when six British divisions of Horne's First Army, including two Canadian, attacked behind tanks a 5-mile front occupied by eleven German divisions and captured Dury Ridge and Quéant, together with 16,000 men and 200 guns. So far, therefore, from Ludendorff's strategic retreat being conducted 'according to plan', it cost the Germans, between 21st Aug. and 9th Sept., some 53,000 men and 470 guns; the French had been able to occupy Ham and Chauny, while the British were going forward; and General Sixt von Armin was forced cautiously to retire from the Ypres salient.
During these operations by the British armies Foch had never relaxed pressure with the three armies of Debeney, Humbert, and Mangin, while still threatening an advance beyond the Vesle in the deflating Aisne salient, west of Rheims, and preparing new blows elsewhere. At the beginning of September the position of the French armies of the centre, won by continuous fighting, was as follows: Debeney had crossed the Somme, taken Ham, and was threatening St. Quentin; Humbert was close to Tergnier and was pointing towards La Fère: Mangin was back in Coucy-le-Château and held the railway thence to Soissons; Degoutte was spreading from Soissons along the Aisne. These threats left General Ludendorff no choice but to shorten his line where he could do so with least risk. He decided on the Vesle front, where General de Mitry, with French and Americans, had been engaging his Seventh and Ninth Armies, and began to retire thence on 4th and 5th Sept. on a 19-mile front. The Americans occupied the Aisne thereupon from Condé to Viel-Arcy, and on 7th Sept. General Mangin crowned his long campaign at the Chemin-des-Dames by taking the ruined Fort de Condé. A week later Allemant and Laffaux Mill fell, and once again the French troops came in sight of Laon. Humbert and Debeney, both pushing forward, embarrassed Ludendorff in his intention of moving divisions to meet a new British movement known thereafter as the battle of Epéhy.
This battle was the preliminary movement in that great attack on the Hindenburg line which,
more than any other single action, was the decisive 'blow at the heart' of the German defensive plan. The British advance, viewed as one movement, was made towards Cambrai, which was the northern bastion of the German defensive lines, as La Fère and Laon were the twin southern pillars. On 2nd Sept. the Third Army began a local attack on a 5-mile front which captured Havrincourt and Trescault, while on the extreme right of the Fourth Army the 9th Army Corps and the Australian Corps began a movement which by 17th Sept. placed them in Maissemy, where the Fifth Army in March had been pierced. These preliminary positions having been secured, the Third and Fourth Armies set in motion their important combined attack (18th Sept.) on a 17-mile front from Gouzeaucourt, through Havrincourt to Holnon Wood, where Debeney's First Army lent assistance. The hardest fighting was at Epéhy, which gives its name to the battle, on the left centre, but by nightfall the German defences had been pierced on a 3-mile front, 12,000 prisoners had been taken, and the British forces brought within striking distance of the main Hindenburg lines.
During the weeks in September while the plans for the battle of Epéhy were ripening, Foch had struck hard at another point in the German line, which had appeared invulnerable while the Germans were strong, but was now a menace to Ludendorff's own plans for retirement because it absorbed divisions which he badly needed elsewhere, namely the long-standing salient of St. Mihiel. It was held by seven German and two Austro-Hungarian divisions in September, and Ludendorff had been withdrawing its heavy artillery; but before his plans for withdrawal could be consummated, Foch sent in General Pershing with his young American divisions, aided by two French divisions, at the salient's apex. The Americans attacked on the two faces of the salient, west and south, the strongest thrust being made by two corps of seven divisions apiece on the southern face. The attack began on 12th Sept., and in thirty hours the salient had disappeared; while in spite of the haste with which the Germans had left it—the firmest resistance was offered by the two Austro-Hungarian divisions on the western face—10,000 prisoners and 450 guns were left behind. This victory, as symptomatic as others of German disorder, freed the Verdun-Commercy railway, and completed the attenuation of Ludendorff's reserves. The 207 German divisions which Ludendorff had commanded at the period of the greatest German strength had fallen to 185, and only 21 reserve divisions remained.