a decisive battle () but French attacks on 18 September, led Falkenhayn to order the 6th Army to operate defensively to secure the German flank. French plan of attack French attempts to advance after the German retirement to the Aisne were frustrated after 14 September, when German troops were discovered to have stopped their retirement and dug in on the north bank of the Aisne. Joffre ordered attacks on the German 1st and 2nd armies but attempts by the Fifth, Ninth and Sixth armies to advance from had little success. The Deuxième Bureau (French Military Intelligence) also reported German troop movements from east to west, which led Joffre to continue the transfer of French troops from the east, which had begun on 2 September with the IV Corps and continued on 9 September with the XX Corps, 11 September with the XIII Corps and the XIV Corps on 18 September. The depletion of the French forces in the east, took place just before the Battle of Flirey, a German attack on 20 September against the Third Army on either side of Verdun, the Fifth Army north of Reims and the Sixth Army along the Aisne, which ended with the creation of the St. Mihiel Salient. Joffre maintained the French emphasis on the western flank, after receiving intercepted wireless messages, showing that the Germans were moving an army to the western flank and continued to assemble the Second Army to the north of the Sixth Army. On 24 September the Second Army was attacked and found difficulty in holding ground, rather than advancing round the German flank as intended. Battle First phase, 25 September – 4 October Battle of Picardy, 22–26 September The French Sixth Army began to advance along the Oise, west of Compiègne on 17 September. French reconnaissance aircraft were grounded during bad weather and cavalry were exhausted, which deprived the French commanders of information. As news reached Joffre that two German corps were moving south from Antwerp, the Sixth Army was forced to end its advance and dig in around Nampcel and Roye. The IV and XIII corps were transferred to the Second Army, along with the 1st, 5th, 8th and 10th Cavalry divisions of the Cavalry Corps (General Louis Conneau), the XIV and XX Corps were withdrawn from the First and the original Second Army to assemble south of Amiens, with a screen of the 81st, 82nd,