[5] Diou volunteered for a five year period of service with the French Army on 22 October 1873 and entered the Ecole Spéciale Militaire de Saint-Cyr two days later.

He was commissioned as a sous lieutenant on 1 October 1875 with the 69th Infantry Regiment (Diou's graduating class was named for Archduke Albrecht, Duke of Teschen who had fought against France's enemy Prussia in the Austro-Prussian War).

[2][6][7] As a subaltern he was reportedly a "shy and timid character, but an energetic worker" and was commended by the Minister of War (Jean Auguste Berthaut) on 31 January 1877 for survey work undertaken in the vicinity of Toul.

[3][8] Diou was promoted to captain in the 45th Infantry Regiment on 29 December 1885 and held responsibility for preparing enlisted candidates for the non-commissioned officer examinations at the Ecole Militaire in Saint-Maixent.

[1][2] Transferred to command the 4th Battalion of the Light Infantry of Africa on 29 December 1897, Diou received praise for his service at Gabès which was acknowledged as a difficult posting.

[8] Diou served in the French possession of Tonkin, Indo-China in 1900–1 and also apparently saw action in the Boxer Rebellion, receiving the 1901 China expedition commemorative medal.

[8] The French commander in Morocco, General Albert d'Amade, praised Diou's actions there stating that he "took part in almost all the engagements of the historic campaign in Casablanca and the outlying regions" and that he was sure to set an "example of calm and constancy of duty" in his new post.

[7] Diou's brigade was mobilised as part of General Louis Émile Taverna's 16th Army Corps and saw action in the Battle of Lorraine in August 1914.

[3] The 143rd Infantry Regiment, part of Diou's brigade, were holding a defensive position in the Mulhewald Woods where they fought off German probing attacks on 19 August.

[9] Diou led a counterattack at 6 am that fared well initially, pushing the German troops back to a wooded area where, favoured by the terrain and the poor light, they were able to mount a strong defence.