He held an important staff position under Joseph Joffre, the French commander-in-chief, at the First Battle of the Marne, before later commanding a corps in the front line.

Appointed a member of the Conseil supérieur de la guerre, he was among the supporters of the decision to build the Maginot Line.

Berthelot was in charge of the Second (Intelligence) and Third (Operations) Bureaux; First Bureau (Personnel & Transport of Materiel) and the Direction de l’Arriere (lines of communication) reported to the Second Sub Chief of Staff (General Deprez, replaced in mid-August by Colonel Maurice Pellé, former military attache in Berlin).

[3] Tuchman wrote that Berthelot was “quick and clever, (and) like his British opposite number General Wilson, was an inveterate optimist.

Sixth Army also launched a diversionary offensive north of Soissons and the Aisne, solid defensive ground where the Germans had halted their retreat in September 1914.

Central Powers forces under General von Falkenhayn had already broken through the Transylvanian passes on 11 November, and swiftly conquered Wallachia and Dobruja.

[13] On the way home from his visit to Petrograd in early 1917, shortly before the Fall of the Tsar, General de Castelnau stopped off for talks with Berthelot, and was told that the Romanian army could not be ready before 15 May.

[14] In August 1917 Foch sent General Albert Niessel (a Russian speaker and formerly commander of IX Corps), on a mission to Russia (by then a republic under the Provisional Government) in the hope of repeating Berthelot’s success there.

[16] The results of the reorganisation and resupply of the Romanian troops were seen in August 1917, when Alexandru Averescu's army broke the front at Mărăşti.

The Central Powers' major counter-offensive under Mackensen, aiming to occupy the rest of Romania (Moldavia) and the port of Odessa, was stopped at Mărăşeşti and Oituz.

At Berthelot's suggestion, Britain and France issued a statement that Romania had fought hard and had been overcome by circumstances beyond her control and that the imposed peace treaty would be disregarded by the Allies.

Godley, the British commander, was angry at being “hustled in, in a great hurry”, without artillery, but later conceded that Berthelot's decision had been correct “in the circumstances” and had been “justified by the event”.

[22] Berthelot praised the bravery of the British XXII Corps (as did Fayolle, commander of Army Group North); privately he thought less well of the British, writing “a certain number of hours’ work, then a rest, and, if it gets too hot, you move further back!”[23] Berthelot was recalled to Paris after Louis Franchet d'Espèrey’s victory on the Salonika front at the end of September 1918, which put Bulgaria out of the war.

[24] He advanced on Bucharest and entered Giurgiu, where streets were named after him and the two French soldiers who died crossing the Danube, on 15 November.

From 1920 to 1926, Berthelot was a member of the Conseil Général de Guerre (Supreme War Council), and was involved in the decision to build the Maginot Line.

Grateful for the French army's contribution to the liberation of Romania, and in particular, Berthelot's role during the World War I Romanian campaign, the Romanian Parliament awarded him honorary citizenship of Romania and King Ferdinand rewarded the general with lands located in the Transylvanian village of Fărcădin, confiscated from the Nopcsa family.