She participated in World War II, first on the side of the Allies from 1939 to June 1940, then in the navy of Vichy France until she was scuttled at Toulon in November 1942.

Propelled on the surface by two diesel engines producing a combined 6,000 horsepower (4,474 kW), they had a maximum speed of 18.6 knots (34.4 km/h; 21.4 mph).

Also called “deep-cruising submarines”, their range on the surface was 10,000 nautical miles (19,000 km; 12,000 mi) at 10 knots (19 km/h; 12 mph).

[3][4] After a stop at Aden, they headed across the Indian Ocean, interrupting their voyage with a visit to Colombo, Ceylon, from 2 to 8 December 1938.

[3][4] On 15 December 1938, the two submarines anchored for the night off Cap Saint-Jacques on the coast of Cochinchina, the southern portion of French Indochina.

During their stay there, Italy invaded Albania on 7 April 1939, and they received orders to prepare for possible combat operations against Italian forces.

[3] With concern for Phénix′s safety growing as the day wore on, L'Espoir transmitted a message at 12:18 to report Phénix missing.

[4] When the news reached the French Commander-in-Chief of the Naval Forces in the Far East, Squadron Vice-Admiral Jean Decoux, at 15:00, he ordered Lamotte-Picquet to get back underway and return to the area to join the search.

[4] Lamotte-Picquet reached the scene at 16:35, and she and L'Espoir searched until 17:06, finding only a slick of diesel oil which had appeared on the surface in the area in which Phénix had disappeared.

[4][6] With Decoux aboard, L'Espoir put back to sea that morning and joined Lamotte-Picquet, the aviso Marne, and the survey ship Octant in the search.

[3][4] Strong currents interfered with the search,[7] but during the morning, a Loire 130 flying boat of Escadrille 5 took advantage of favorable lighting conditions to spot Phénix′s wreck, which lay on the seabed in 105 metres (344 ft) of water.

[4] At the start of World War II in September 1939, L'Espoir was assigned to the 5th Submarine Division in the 1st Flotilla of the 2nd Squadron, based at Toulon.

When the attack on Mers-el-Kébir — in which a British Royal Navy squadron attacked a French Navy squadron moored at the naval base at Mers El Kébir near Oran on the coast of Algeria on 3 July 1940 — took place, she was a part of Group A at Toulon along with her sister ships Archimède and Le Conquérant.

[3] The three submarines received orders that day to form a patrol line during the night of 6–7 July 1940 to protect Oran and attack British ships, the line to extend from north to south in the Mediterranean Sea for a distance of 20 nautical miles (37 km; 23 mi) east of Alboran Island and south of Cape Palos.

[3][12][13] On 11 October 1940, L'Espoir and Vengeur departed Toulon bound for Oran in company with the tanker Lot, which was to accompany the four submarines on their journey to French Indochina.

[12] Subsequently, the submarines took turns conducting defensive patrols 10 to 35 nautical miles (19 to 65 km; 12 to 40 mi) off Dakar.

[12] On 17 December 1940, Lot and the four submarines got underway from Dakar to continue the voyage to French Indochina, with their next stop at Madagascar.

[3] L'Espoir next conducted an Indian Ocean cruise in which she called at Pointe des Galets on Réunion from 26 to 29 April 1941 before proceeding to Mauritius to reconnoiter Mauritius′s southern tip.

[3] While at sea, she received orders to come to the assistance of the French cargo ship Charles L. D., which had been captured between Réunion and Madagascar, but her efforts were unsuccessful.

[3] Operating on only one diesel engine, L'Espoir left Diego-Suarez on 16 August 1941 for courtesy visits to ports along the coast of Madagascar and to the northwestern Comoro Islands.

[3] The submarines made an overnight stop at Tulear, Madagascar, on 21–22 March 1942, where they met the French auxiliary cruiser Quercy.

[3] They next stopped at Dakar from 19 to 27 April, at Casablanca in French Morocco from 3 to 8 May, and at Oran from 10 to 11 May before arriving at Toulon on 13 May 1942,[3] where they began repairs.