He commanded a corps at the Battle of the Somme in 1916 then, in the second half of 1917, served as chief of staff to the French Commander-in-Chief Philippe Pétain.

[5] On 2 June 1917 Debeney met Field Marshal Sir Douglas Haig (Commander-in-Chief of the British Expeditionary Force) and told him that the French would still participate in Haig's upcoming Flanders offensive (First Army, now under Anthoine, moved north on 7 July, ready to provide the promised six divisions along with artillery and air support), but that the planned attack on the Chemin des Dames by Sixth Army, badly affected by the mutinies, was to be cancelled.

Debeney estimated that a delay of a month would be needed to allow the troops rest and leave and to restore morale.

Despite his later claims to have been motivated throughout the second half of 1917 by concerns about the state of the French Army, there is no evidence that Haig was particularly disturbed by this news.

Debeney commented that the Allies could never have beaten Napoleon without a joint command, "even though he was an idiot" (Sidney Clive notebooks 15 August and 3 September).

[10] In September and October 1917 Debeney joined Pétain in blocking general staff proposals to attack in the difficult terrain of Alsace.

The note demanded not just defence in depth but also the construction of bretelles (trenches perpendicular to the front lines to halt lateral exploitation in the event of a German breakthrough).

All commanders were required to set labour works in motion immediately and to submit plans to GQG by 15 December.

[17][18][19] Debeney’s Army linked up with the Australian 35th Battalion to plug the gap at Villers-Bretonneux, the last high ground in front of Amiens.

The BEF unit on First Army's immediate left was a Canadian armoured car detachment under the French-born Brigadier-General Raymond Brutinel.

On the right Humbert's Third Army, which had borne the brunt of the Battle of the Matz in June, was to join in on 10 August, as soon as Debeney had made enough progress, an aspiration thought absurdly optimistic by Petain's staff.

In the amended version of his diary he claimed that Debeney was “much distressed and almost in tears” because three battalions of Colonial Infantry had “bolted” before German machine gun fire.

That day Foch urged Debeney to continue to attack “with drums beating” and to "go quickly, straight ahead, manoeuvre, push from behind with all you have until you obtain a decision".

That evening X and XXXV Corps (from the south) encircled Montdidier and that night German Eighteenth Army withdrew to north of Roye.

After a visit to Haig's GHQ Foch ordered First Army to shift its efforts north of St Quentin, but the British were still complaining about the French "hanging back".

[44] On 10 October, already aware from French intelligence that Germany had extended armistice feelers to the US, Foch ordered Debeney's First Army to extend its front northward and to overrun the line of the River Serre, to enable the British to concentrate their efforts at the liberation of Lille (which was liberated on 17 October) and between the rivers Sambre and Scheldt/Escaut towards Mons.

After the war he was Director of the École de Guerre, where he introduced a new curriculum, and often had his students “walk the ground” of his 1918 campaigns.

He was commandant of La Place de Paris and a member of the commission which wrote France's postwar doctrinal manual: “Instruction provisoire sur l’emploi tactiques des grandes unités” (1922).

While France was to be defended by the Maginot Line and her network of allies in central Europe, the plan was for the main French army to defeat a German incursion through Belgium by bataille conduite (methodical battle).

[47][48] A statue of Foch stands on the Bapaume-Peronne road, near the village of Bouchavesnes, at the point where Messimy’s chasseurs broke through on 12 September 1916.

Debeney described the statue at its unveiling in 1926 as “the effigy of the conqueror of the Somme”, who had “reawakened the spirit of the offensive in our army and given it the confidence that success would follow from careful preparation and bold execution”.

The Somme was “the first of the great massed battles, in which we asserted our tactical superiority over the enemy … After the Somme (Foch) began to abandon the simplistic idea of obtaining success by breaking a short section of the enemy’s front, replacing it with the more fruitful idea, which was to give us victory, of progressively dislocating the various sectors of the front”.