[2] After the First World War began in August 1914, the 1st, 4th and 5th Destroyer Flotillas were tasked to escort the core of the 1st Naval Army while the 2nd, 3rd and 6th Flotillas escorted the armored cruisers of the 2nd Light Squadron (2e escadre légère) and two British cruisers during the preliminary stages of the Battle of Antivari on 16 August.

After reuniting both groups and spotting the Austro-Hungarian protected cruiser SMS Zenta and the destroyer SMS Ulan, the French destroyers played no role in sinking the cruiser, although the 4th Flotilla was sent on an unsuccessful pursuit of Ulan.

Four days later, the fleet covered the evacuation of Danilo, Crown Prince of Montenegro to the Greek island of Corfu.

The flotilla escorted multiple small convoys loaded with supplies and equipment to Antivari, beginning in October and lasting for the rest of the year, always covered by the larger ships of the Naval Army in futile attempts to lure the Austro-Hungarian fleet into battle.

[3] In October 1915, Mortier and the destroyer Hache supported the predreadnought battleships Saint Louis and the Charlemagne as they covered landings by French troops in Salonica, Greece.