It was General Joffre's plan to retreat to a position south of the Marne, where his reserves would be available, a movement which was successfully carried out by all parts of the allied line during the following week. By September 5, 1914, this line extended from the southeast of Paris, along the southern tributaries of the Marne, across the Champagne to a point south of Verdun. Beyond that, De Castelnau was still holding the heights in front of Nancy. The powerful German advance had forced the Allies back some hundred and thirty miles, almost to the shelter of the Paris fortifications. It seemed only a matter of hours to the fall of Paris when General Joffre began his counteroffensive on September 6, 1914. Attempting to pierce and envelop the allied left center, Von Kluck marched across the front of the British to strike at the Fifth French Army commanded by General d'Espérey, who had replaced Lanrezac after the Charleroi defeat. But the turn of the tide was at hand. The Sixth French Army from Paris, under General Manoury, fiercely attacked Von Kluck's rear guards on the Ourcq; Sir John French drove against the right of the main German advance; the Fifth and Ninth French armies held the front of Von Kluck and Von Bülow; the Fourth French Army south of Vitry resisted the piercing movement of the Duke of Württemberg, and the Third French Army (General Sarrail) checked the crown prince at Verdun, while De Castelnau at Nancy entered upon the final stage of the battle of Lorraine. The first great German offensive had failed in its purpose. By September 12, 1914, the whole German front was retreating northward. The Aisne plateau, where the Germans came to a halt, is considered one of the strongest defensive positions in Europe, and General Joffre soon realized that it could not be taken by direct assault. He therefore attempted to envelop the German right and extended his left wing--with a new army--up the valley of the Oise. Some desperate German counterattacks were met at Rheims and south of Verdun, but they achieved small success beyond creating a sharp salient in their line at St. Mihiel, where the invaders managed to cross the Meuse, General Sarrail defended Verdun with a field army in a wide circle of intrenchments, with the result that the crown prince was unable to bring the great howitzers within range of the fortress, and his army suffered a severe defeat in the Argonne.