stalled Second Army. Critically, the move exposed Kluck's right flank in the direction of Paris where (unknown to Kluck) General Michel-Joseph Maunoury's new Sixth Army was deployed. The French learned of Kluck's change in course on September 3 thanks to reports from Allied aircraft, and this was independently confirmed by radio intercepts. The following events were critical to the future course of the war. On 5 September, Maunoury attacked Kluck's right (west) flank, marking the opening of the First Battle of the Marne. Kluck parried the blow by borrowing two corps in the space between the First and Second army. A surprise attack on 8 September by Franchet D'Esperey's (who had replaced Lanrezac) Fifth Army against Bülow's Second widened the gap which the British Expeditionary Force marched to exploit. Von Kluck telegraphed von Moltke on the night of 8 September that the decisive victory would be won the following day. Instead, on 9 September a representative of the German Headquarters, Hentsch, considered the situation of Bülow's Army as very dangerous and ordered a retreat of all the armies, even though by that time von Kluck had overcome most of his own problems. Ian Senior dismisses as a "myth" the claim in the German Official History that von Kluck might have triumphed on 9 September. In fact the BEF was already over the Marne and Quast's attack against Maunoury's Sixth Army had failed, and suggests that this may be why Kluck avoided meeting Hentsch directly. The Germans retreated in good order to positions forty miles behind the River Aisne. There, the front would remain for years in the form of entrenched positions as World War I continued. Kluck and Bülow's lack of coordination and the ensuing failure to maintain an effective offensive line was a primary contribution to the failure of the Schlieffen Plan which was intended to deliver a decisive blow against France. Instead, the long stalemate of trench warfare was ready to begin. Many German experts hold Kluck and especially his chief of staff, Hermann von Kuhl, in the highest esteem. Germany could have won the Battle of the Marne, they think, if only Bülow had matched the courageous initiatives of Kluck's Army, although this doesn't explain the near encirclement of his army. The British at the time called him "old one o'clock". “In great and dangerous operations one must not think but act”, was von Kluck's favourite quote