The Allied plan was now fully revealed. Instead of presenting on the Grand Morin an ironclad face, safe in flank and rear, the I German Army had been suddenly thrown on to a rectangular defensive on a front of 50 miles between Betz and Courtacon, against attacks converging from the west, south-west, and south. That evening, at Joffre’s request, the British line was directed more to the north, thus emphasising the effect of Maunoury’s move. From this moment, the withdrawal of the whole of Kluck’s forces over the Marne must have been envisaged. On the following day, September 7, in fact, the III and IX Corps (west of Montmirail), were preparing to follow the IV Active Corps across the Marne; but the Allies were then aware of what was happening. Marwitz’s Cavalry Corps covered the movement along the Grand Morin, with one division to the west, one to the east, and one 4 miles north of Coulommiers, while Richthofen’s Divisions operated farther east, all available artillery supporting them. The task was fulfilled with much resource and energy; but the position was not one that could be long maintained, for the British 3rd Corps was at Maisoncelles, 4 miles beyond the Grand Morin, and the French 8th Division threatened the German flank at double this distance northward by occupying St. Fiacre and Villemareuil. At noon, Marwitz gave way, falling back to the Petit Morin, from La Ferté-sous-Jouarre south-eastward. By evening, the British 3rd and 2nd Corps were beyond the Grand Morin at La Haute Maison and Aulnoy; the 1st was held back somewhat from Chailly to near La Ferté-Gaucher, in touch with the French 5th Army. General de Lisle’s Cavalry Brigade, with the 9th Lancers and the 18th Hussars, showed especial vigour. The men were full of cheer, and ready for anything; but Sir John French was a careful commander. The measure of the enemy’s retreat could not be immediately taken through the curtain of cavalry and artillery——aviation was in its infancy in those days. All the strength available was in line; and it was so thin a line as to tempt surprise. The Field-Marshal considered the alternative of sending direct help round to Maunoury, but concluded that the best aid would be to drive rapidly to and across the Marne.[61] III. A RACE OF REINFORCEMENTS