The open western flank beyond the 1st Army and the danger of attacks from the National redoubt of Belgium, where the Siege of Antwerp had begun on 20 August, created a dilemma in which the German positions had to be maintained, when only offensive operations could lead to decisive victory.

[10] On 10 September, Joffre ordered the French armies and the British Expeditionary Force (BEF) to advance and exploit the victory of the Marne.

For four days, the armies on the left flank advanced and gathered up German stragglers, wounded and equipment, opposed only by rearguards.

From 11 and 12 September, Joffre ordered outflanking manoeuvres by the armies on the left flank but their advance was too slow to catch the Germans.

The main German effort remained on the western flank, which was revealed to the French by intercepted wireless messages.

On the right flank, the French 17th and 45th divisions attacked near Soissons and gained a foothold on the plateau of Cuffies, just north of the city.

The new French Second Army prepared to begin an advance on 22 September, on a line from Lassigny north to Roye and Chaulnes around the German flank.

[14] On 21 September, Falkenhayn met Bülow and agreed that the 6th Army should concentrate close to Amiens and attack towards the Channel coast and then envelop the French south of the Somme in a Schlachtentscheidung (decisive battle).

[18] The German XXI and I Bavarian corps recaptured Péronne and forced the Second Army west of the Somme, where the French managed to dig in on good defensive ground, from Lassigny to Roye and Bray.

[19] The French had used the undamaged railways behind their front to move troops more quickly than the Germans, who had to take long detours, wait for repairs to damaged tracks and replace rolling stock.

[18] The French had been able to move troops in up to 200 trains per day and use hundreds of motor-vehicles, which were co-ordinated by two staff officers, Commandant Gérard and Captain Doumenc.

[21] Until the end of the Siege of Maubeuge (24 August – 7 September), only the single track from Trier to Liège, Brussels, Valenciennes and Cambrai was available.

The French resorted to more cautious infantry tactics, using cover to reduce casualties and a centralised system of control, as the German army commanders followed contradictory plans.