This is a somewhat modified version of a previous post on this topic, found here. In the run-up to the war, the British and French agreed to a split in naval responsibilities. In the event of war with Austria-Hungary and Germany, the British were to operate against Germany in the North Sea, while the French concentrated on the Mediterranean. In general, this was the case throughout the war.

When the war began, the majority of the French fleet was in the Mediterranean, under the command of Vice Admiral Augustin Boué de Lapeyrère. While the RN's Mediterranean Squadron moved to chase the German battlecruiser Goeben and the accompanying light cruiser Breslau, Lapeyrère took his fleet to blockade the Strait of Otranto. On the 16th August, he took his fleet into the Adriatic to support Montenegro. The aim of the sortie was to clear the Austrian blockade of the Montenegrin port of Antivari (now Bar), letting the Allies ship supplies to Serbia and Montenegro. His force encountered the Austro-Hungarian light cruiser Zenta, and overwhelmed it. Following this, the French fleet set up a rolling blockade of the southern Adriatic. The fleet was based at Malta, some 550 miles away. To ease the logistical pressure this created, Corfu was used as a forward base, though this breached Greek neutrality. Lapeyrère made several sorties to attack the Austro-Hungarian base at Cattaro (Kotor), bombarding it on regular occasions. Some of his cruisers even travelled further up the coast, albeit to little effect. He also covered supply convoys running to Antivari to support the Montenegrin Army. The French fleet in the Adriatic went effectively unopposed until late 1914, when Austro-Hungarian submarines began to be based at Cattaro. On the 21st, U 21 would torpedo Lapeyrère's flagship, the modern dreadnought Jean Bart. While she would survive, the torpedoing put her out of action for several months, and caused the French to pull their battleships out of the Adriatic. This encouraged the Austro-Hungarians to be bolder with their light forces, laying mines in the southern Adriatic and raiding the area with submarines and destroyers. On the 26th-27th April, the submarine U 5, commanded by Georg von Trapp of The Sound of Music fame, sank the French armoured cruiser Léon Gambetta. This caused the French to effectively pull back their entire fleet from the Adriatic, at least until Italy's entry to the war.

The Italian entry resulted in a rather convoluted command structure. Lapeyrère (and his successor, Admiral Fournet) was still in overall command of the Allied forces in the Mediterranean, with the Italian Admiral Luigi of Savoy, Duke of the Abruzzi as his second-in-command. However, if Lapeyrère took his fleet into the Adriatic, he would come under Abruzzi's command. Following this, the French played a comparatively minor part in the war in the Adriatic. Their largest contribution was the evacuation of the Serbian Army from Albania. They also supported the Otranto Barrage, the Allied attempt to block submarines from travelling through the Strait of Otranto, and participated in later sorties into the Adriatic.

The French Navy also operated in the Eastern Mediterranean. In 1914, a French squadron joined the British force guarding the Dardanelles against a possible breakout by Goeben and Breslau. Battleships from this force would join the initial British bombardments of the Dardanelles forts. Following this, the Allied force entered the straits on the 18th March, with four French battleships joining the sixteen British ships. Unfortunately, they would encounter mines, and the French battleship Bouvet would be sunk, along with two British ships.After this attempt, the French Navy assisted the RN in supporting the landings on the Gallipoli peninsula. French submarines joined British and Australian subs in the Sea of Marmora, while French cruisers guarded the rest of the Ottoman coast. After the evacuation from the Dardanelles, a French squadron formed the main force watching the straits.

The French Navy was instrumental in operations on the Salonika Front. The Allies had transported a small force to the Greek port of Salonika (Thessaloniki) to take the pressure off the Serbs, albeit to little effect. Once the Serbs were forced out of Serbia, they were also transported to Salonika. The Allies were allowed to do this thanks to the political crisis ongoing in Greece at the time. The Greek king, Constantine I, was sympathetic to the Central Powers, and hence favoured Greek neutrality, while Greece's Prime Minister, Eleftherios Venizelos, was pro-Allied. Venizelos invited the Allies to use Salonika, against the wishes of Constantine. Tensions between the two came to the head in the winter of 1916. In November, the French forced the Greek Navy at Salamis to disarm itself, and commandeered many of their ships. This led to significant opposition from Constantine and his supporters. Loyal army units and Royalist militias began to occupy Athens to prevent the Allies deposing him or forcing Greece into the war. A French naval squadron arrived off Piraeus on the 1st December. Armed sailors and marines were landed, and the city bombarded; ultimately, Constantine would be forced to agree to Allied demands. Shortly afterwards, though, the battleship Gaulois would be sunk by a German submarine near Salonika.

The French operated outside the Mediterranean, albeit to a lesser extent. French forces in the Far East participated in the hunt for German surface raiders, with the destroyer Mousquet being sunk by Emden during her raid on Penang. French forces helped to escort convoys in the Atlantic, and several ships were lost there. The battleship Suffren and armoured cruisers Kléber and Dupetit-Thouars were all sunk by German submarines in the Atlantic.