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. 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. Diou and Colonel Berguin, commander of the regiment, were commended for their coolness and courage under shellfire in this action. Diou's brigade was ordered to attack German positions at Bénestroff the next morning at 4 am in heavy fog. Before the attack could begin a German offensive launched from Cutting, Loudrefing and Mittersheim hit the regiment, supported by heavy artillery fire. The 143rd's 2nd battalion was able to hold off the attack long enough for the 1st and 3rd battalions to gain defensive positions before withdrawing to join them. 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. Several charges were made by the French troops but they were eventually forced to withdraw under heavy defensive fire. During this attack Diou, who led from the head of his troops with gun in hand, fell wounded in action. His brigade's second in command, Commandant Jacques, was later killed in action in the same engagement and the 143rd also lost Colonel Berguin and most of their medical staff. Diou died of his wounds on 23 August 1914 at Dieuze.