The battle of Bapaume, 21 August-3 September, was the second phase of the battle of Amiens, the British offensive often taken to be the turning point of the First World War on the Western Front.
The first phase of the battle had seen the British Fourth Army push the Germans back ten miles, from the tip of the Amiens salient created during the first of Ludendorff’s summer offensives (second battle of the Somme), back in part to the front line they had held before the first battle of the Somme. The allied commander-in-chief, Ferdinand Foch, wanted the Fourth Army to launch an immediate attack on this line, with the aim of pushing the Germans back to the Somme.
Haig believed that the new German position was too strong to attack without careful preparation. The old Somme battlefield had been fought over in 1916, deliberately devastated by the Germans in 1917 and then fought over again in 1918, and was not well suited to tank warfare. It would need a heavy artillery bombardment to destroy the wire, and it would take time to move the artillery forward.
Haig preferred to launch a new offensive further north, using the Third Army (General Byng), supported by 100 tanks, to attack the German Seventeen Army (Marwitz) across more suitable ground. Foch agreed to Haig’s plan, although he did remove the French First Army from British control. It would launch its own offensive on the same day as the renewed British attack.
During the relative lull between 11 and 21 August, Byng’s Third Army was reinforced, while the Canadian Corps moved from the Fourth to the First Army (Horne), on the left of the line.
The British attack began on a narrow front on 21 August, with an attack by the Third Army. The Germans responded with a counterattack on 22 August, which was quickly beaten off. On 23 August Haig was able to order a general advance by the Third Army and part of the Fourth, on a 33 mile front. On 26 August the right wing of the First Army joined in, extending the front to 40 miles (this attack is sometimes designated as the second battle of Arras of 1918). At that point the German line ran along the Somme south from Péronne, then across open country to Noyon on the Oise. Ludendorff had ordered a retreat from the Lys salient and what was left of the Amiens salient, with the intention of forming a new line on the Somme.
This plan was disrupted by the Australians, New Zealanders and Canadians. On 29 August the New Zealanders captured Bapaume (east of Amiens, south of Arras), breaking through the Le Transloy-Loupart trench system.
To the south the 2nd Australian Division captured Mont St. Quentin, on the east bank of the Somme, on the night of 30-31 August, and on 1 September captured Péronne itself.
Further north, on 2 September the Canadians broke through the Drocourt-Quéant switch, a strong section of the German line south east of Arras. With two gaps in the proposed new front line, Ludendorff was forced to retreat back to the Hindenburg line, abandoning all of the territory won earlier in 1918.