The early part of the night between the 8th and 9th of September was clear and starry, but later the weather broke; the rain came down in torrents, and soon the marshes of the Gond were wellnigh impassable for wagons and guns. Nothing could have been more fortunate for General Foch. He knew that, as the German right was in retreat, he might safely strike at the right of the army which was facing him, and thus drive in a wedge between von Kluck and von Buelow. He advanced towards the Petit Morin with the Morocco Division, and, deploying in the direction of the Marshes, met a furious assault of the Germans, who now perceived the perilous position in which they were placed. The Moroccans fought like heroes, and drove the Germans into the Marshes, where they found that they could not move their guns or wagons, which were up to the axle-trees in mire. Foch captured many prisoners and at least forty guns--the largest number which had so far been taken at one time by the Allies.
History had repeated itself. In the campaign of 1814 German troops had been driven into the self-same swamp; a hundred years later a similar disaster had overtaken them. This success greatly elated Foch's army, and it was eager for the next move.
A wedge had been driven between von Kluck and von Buelow; it now remained to drive a similar wedge between von Buelow and von Hausen, whose forces continued the German line to their left. Foch's airmen had told him that there was a considerable gap between the left of von Buelow and the right of von Hausen. He now moved the division which had driven von Buelow's right into the Marshes to the right of his line, and having thus reinforced it, made a flanking movement on the left of von Buelow and on the right of von Hausen. It was one of the boldest moves ever made, and it took the enemy completely by surprise. The German Guard made but little resistance. Von Buelow knew that he was outflanked on both sides, and that there was no safety except in retreat. Foch followed him up, and on the 11th drove him across the Marne in disorder. The losses of the Germans in this part of the retreat were enormous; it was said that the 9th Army buried ten thousand German dead.