The help rendered by the Russians at this critical phase of the war was invaluable, and played no small part in the approaching struggle on the Marne. In his fears for the safety of Paris, Joffre was naturally anxious to profit by this relief, and discussed with Sir John French the possibility of taking the offensive at the earliest possible moment. There appears to have been some misunderstanding as to Sir John's plans at this point. The British Commander-in-Chief declares that he had every intention of remaining in the line and filling the gap between the French Fifth and Sixth Armies, but the French Higher Command was apparently under the impression that he was determined not to fight any more until his troops had been given a week to reorganize and refit. Lord Kitchener himself hurried to Paris to clear the matter up, but "full accord", according to President Poincaré, long afterwards, "was not re-established without trouble". As soon, however, as the offensive was ordered, continued the same authority, the British Commander-in-Chief gave his assistance without reserve. "His army fought with magnificent courage, and Great Britain played a brilliant part in the common victory."
In the meantime the retreat continued, the British, on 2nd Sept., reaching the line of the Marne towards Lagny and Meaux, with the French Fifth Army, now under the command of Franchet d'Esperey, on their right, retiring on Château-Thierry, and Maunoury's new Sixth Army, on their left, retiring towards Paris. It was at this point that von Kluck made the fatal mistake of dismissing the British army as practically crushed and out of action. Diverting the advance of the German First Army, he left Paris on his right in order to deal what he hoped would be a decisive blow at the French Fifth Army south of the Marne. By 5th Sept. the British army had fallen back to the Forest of Crécy to bring it in line with the French Fifth Army.
Not only was the British army at length receiving sorely needed reinforcements, but the French army was every moment increasing in strength and numbers as it fell back on its reserves. Besides the French Sixth Army on the British left, another new French army had sprung into being behind the marshes of St. Gond--the Ninth, under Foch, who filled the gap between Franchet d'Esperey's Fifth and Langle de Cary's Fourth Army--behind Vitry. Eastward the line was continued by the French Third Army, now commanded by Sarrail in place of Ruffey; and Castlenau's Second Army, now fighting the battle of the Grand Couronné de Nancy which stemmed the German invasion at this point, and prevented the threatened envelopment on the Allies' right, where the Kaiser himself had gone to inspire the troops of Prince Rupprecht of Bavaria.