The First Battle of the Marne was won not, as many believed, by any single exploit, but by the faithful performance of its duty by each section of the long-drawn line. Let us look first at the French right flank in Lorraine. There the battle began on the 4th September, and three days later came the crisis when, by the slenderest margin, the enemy failed to break Castelnau on the ridge called the Grand-Couronné. The Kaiser himself was a spectator of the fight, for Germany had counted on forcing the pass; but by the 8th she had failed, and by the 9th Castelnau had firmly barred the gate.
The French centre, under Foch, Langle de Gary, and Sarrail, had a longer period of trial. Sarrail, at Verdun, was all but broken on the 8th, and was compelled to fall back to the west bank of the Meuse. All through the 9th and 10th the desperate struggle continued, and by the evening of the last day the French general was preparing for retreat. Suddenly, however, he found the attack ebbing, and by the 12th the enemy was mysteriously withdrawing. Farther west Langle de Gary had his worst moment on the 8th; on the 9th he received reinforcements which eased his position, and on the 10th he too felt the strange weakening of the enemy. The left centre under Foch had the sternest fight of all. He had against him the bulk of Bülow's and Hausen's armies, and on the 8th he found his flanks turned and his whole front split into gaps. Nevertheless he prepared to attack on the 9th with his last ounce of strength. All that day his centre and right were falling back before the enemy's thrust, but he still persevered in his purpose and marched the single division he could muster to the point where he thought he could strike with the greatest effect. The blow was never delivered, for on the evening of the 9th the apparently triumphant advance halted and ebbed. Like Sarrail and Langle de Gary, Foch, having resisted to the limit of human endurance, discovered that the enemy was miraculously disappearing.
The cause of the miracle was the doings of the French left wing. Joffre had hurled Maunoury on Kluck's flank and rear, while Sir John French and Franchet d'Esperey attacked in front. Kluck met the threat with vigour and resolution. He formed front to flank, as the phrase goes--that is, he faced round to what had been his wing--and in the three days' fighting all but defeated Maunoury. On the night of the 7th the outflanking French left found itself outflanked in turn, and its attack turned into a desperate defence. But on the 9th came salvation. Kluck's manoeuvre had left a gap of 30 miles between himself and Bülow, and into this gap were pouring the British force and that of Franchet d'Esperey. Suddenly Maunoury discovered that certain villages in front of him were evacuated, and his airmen told him of enemy convoys moving to the north. At 1 p.m. that day Bülow began his retreat, and Kluck was forced to follow suit. Sir John French and Franchet d'Esperey had pierced the enemy front, and the retreat of the German right caused the retreat of all the German armies. They fell back to a line along the Aisne, through Champagne, and down the east bank of the Meuse--a strong line, which for four years was never really broken. But, none the less, it was a retreat.