The retreat from Mons, by both the French Army and the British Expeditionary Force ended at the River Marne, approximately 30 miles from Paris. Faced with a counter-attack along the Marne, the Germans slowed down their advance. The French Military Governor of Paris, General Joseph Gallieni, secured overall command of the B.E.F., after consulting with Lord Kitchener. Gallieni had six thousand French reserve infantry troops transported to the battle by approximately six hundred Paris taxicabs. Battle commenced at noon on the 5th September, when the French 6th Army stumbled on the advance guard of the German 1st Army.
The British avoided joining the battle until the commander of the German 1st Army, General Alexander von Kluck, made a tactical error on the 9th September 1914. Von Kluck ordered his forces to pursue and over-run the French 6th Army, retreating to the Marne. A 50km gap opened up between the German 1st and 2nd Armies, and the Allied forces quickly attacked the open flanks of both German armies. The combined French 5th Army and the B.E.F. exploited this tactical error.
Upon learning about this error, the German Chief of General Staff Helmuth von Moltke suffered a nervous breakdown. His subordinates assumed command of the 1st and 2nd Armies, ordering them to withdraw to the Aisne River.
The German retreat, between the 9th to 13th September, effectively caused the abandonment of the Schlieffen Plan. The Schlieffen Plan was designed to by-pass the Allied armies and enter Paris, ensuring France would sue for peace, allowing the German army to concentrate on the Eastern War with Russia. The aftermath of the battle, despite all the enormous efforts by the German forces, had come to nothing.
.The Allies were now pursuing the retreating Germans, and forced both sides to dig trenches on the banks of the river Aisne, which was to be the next major engagement