In the first Battle of the Marne 6–12 September 1914, the Allies reversed their headlong retreat to the River Marne, with a bold counterattack on the advancing Germans’ right flank. In response, the German 1st and 2nd Armies drifted apart creating a breach that the British Expeditionary Force and French 5th Army were quick to exploit. Rattled, the divided Germans were forced to retreat, sacrificing most of their gains in the preceding month before digging in beside the River Aisne. Their defensive position, on a ridge commanding the northern bank of the river, was formidable. Repeated Allied assaults were repelled by withering artillery fire, and soon each side tried to outflank the other in the so-called ‘race to the sea’. These attempts failed and this brief mobile phase of the war on the western front was effectively over: the gruelling war of attrition which was to define the conflict had begun.