He impressed Joffre by presenting a plan for a concerted attack by the Allied armies on 6 September if Maunoury's Sixth Army reached a certain position on the Ourcq at a certain time ("if not the British will not march"). If not, he would retreat a little further, south of the Grand Morin with the Sixth and the BEF l, striking Alexander von Kluck's 1st Army in flank. That was to become the basis for "Instruction Générale No 6", the Allied plan of attack at the Marne.