"Desaix" and "Kléber" were reactivated in July 1914 and were assigned to the 3rd Light Division ( (DL)) of the 2nd Light Squadron which was tasked to defend the English Channel in conjunction with the British. The division was on station in the western end of the Channel by 4 August, where their mission was to intercept German shipping and provide distant cover for the smaller ships escorting convoys in the Channel. Improved defenses in the Channel and the stabilization of the front in early 1915 allowed the cruisers to be released from their tasks, so "Desaix" was assigned to the 3rd Squadron upon her arrival in the Eastern Mediterranean on 16 February. The squadron was tasked to patrol the area between Port Said, Egypt, and Alexandretta, Ottoman Syria. The cruiser was detached in May to assist the 1st Naval Army () in the Central Mediterranean with searching for German shipping near Italian ports. She rejoined the 3rd Squadron and was tasked to help blockade the Ottoman coast near the Turkish and Syrian border. In contrast to her sister, "Kléber" was transferred to the Dardanelles to support Allied forces in the Gallipoli Campaign in May where she joined the Dardanelles Squadron together with "Dupleix" and were assigned to blockade the coast of Asia Minor. "Kléber" collided with the Royal Australian Navy troopship HMT "Boorara" in the Aegean Sea on 17 July, damaging her bow. After the Kingdom of Bulgaria joined the Central Powers in mid-October, "Kléber" was one of the ship tasked to raid the Aegean coast of Bulgaria on 21 October.