In 1922, the Washington Naval Treaty imposed a ten-year moratorium on the construction of new battleships. France was allowed to replace two old battleships after 1927 for a total of 70,000 tons.
In 1925, the first Italian Trento-class cruiser was laid down, and launched the following year; these ships could threaten French sea lanes between metropolitan France and colonial holdings in North Africa. In response, Vice Admiral Henri Salaun of the French Admiralty began discussing counters in 1926 and 1927. They considered a 17,500 ton design which would allow for four ships under the treaty limit. They would have two quadruple 305 mm (12-inches) turrets forward. The use of quadruple-gunned turrets had been planned for the cancelled Normandie and Lyon-class battleships, while the all-forward configuration was inspired by the British Nelson class. Speed would have been 34–35 knots, with armour to resist 203 mm shells. However, these ships were not ordered as they could not withstand old Italian battleships.
The 17,500 ton design was followed by new studies for a 37,000 ton battlecruiser in 1927–1928. A trial displacement of 37,000 tons was equivalent to a standard displacement of 32–33,000 tons, which was close to the treaty limit for battleships. At least two draft designs were made, showing an enlarged Suffren-class cruiser with a 254m long hull with a 30.5m beam and two raked funnels. The main armament's fire control director was atop a tripod foremast. Armament was three quadruple 305 mm turrets (two fore and one aft), eight single 90 mm Mle 1926 HA guns, twelve 37 mm Mle 1925 single AA mountings, and triple torpedo tubes. The armour belt would have a 220–280mm thickness. A hangar was behind the bridge structure with two catapults abeam the second funnel. Power would have been from two groups of boilers and turbines, as on the Duquesne classes, although with 12 rather than 8 Guyot-du-Temple boilers for 180,000 hp and 33 knots.
In 1928, an alternate 37,000 ton design emerged as a fast battleship. This design had a heavier armament of three twin 406mm turrets and four quadruple 130mm turrets. The hull was slightly shorter at 235m, but had a slightly wider 31m beam. Armour was thicker, but had only two-thirds the power for only 27 knots.
The 37,000 ton battleships also did not go forward. The French Navy did not have a dock large enough to build a 35,000 ton hull that was longer than 250m. For comparison, the 247m long SS Île de France had been built at the civilian Louis Joubert Lock at Saint-Nazaire. Building the required naval infrastructure would have cost the same as the two battleships, and interfered with existing building programs. Furthermore, more stringent naval restrictions were being discussed. The League of Nations' Disarmament Subcommittee had turned their attention to naval limitations, and the British were pushing for a limit of 25,000 tons with 305mm guns for battleships. The French did not want to jeopardize these negotiations.
Vice Admiral Violette, Chief of Staff of the French Navy, ordered a new study in 1929 for a protected cruiser from the Service Technique des Constructions Navales, the department responsible for naval construction. The resulting design had a displacement of 23,690 tons, three 305mm turrets (one triple and one quadruple forward, and one triple aft), four twin 138mm mountings (calibre used on the most recent destroyers), and eight twin 100mm turrets for AA (as on Algérie, the latest heavy cruiser). Speed would have been 29 knots,with armour only able to withstand 203mm shells. The design resembled Algérie with a distinctive forward tower with a single funnel abaft. This profile would carry into the final Dunkerque design.
French plans were upset when the Germans laid down the first Deutschland-class cruiser in February 1929. Although the cruisers displaced far more than allowed by treaty, they were nonetheless well-suited for commerce raiding and could outrun all major capital ships except for HMS Hood and the two Renown-class battlecruisers of the British Royal Navy. The French immediately prepared drafts for a counter to the Deutschlands. Protection against the German's 280 mm shells was the paramount requirement, although it seemed the armament from previous projects and a speed of 30 knots might be maintained. The end displacement was 23 000 to 25 000 tons, which fitted within the limitations proposed by Britain.
The 1930 London Naval Treaty extended the "battleship holiday" from the Washington Naval Treaty to the end of 1936, although France and Italy were allowed to lay down new battleships in accordance with the older treaty. However, France refused to adhere to new restrictions, mainly related to cruisers, due to new German naval construction. In response, Italy also refused to accept the new restrictions. Bilateral negotiations between France and Italy took place in early 1931 to resolve the issue, resulting in an agreement on March 1 allowing each country to build only two 23,333 ton battleships until 1936. It was not possible to go further; the Italians were not satisfied with their 23,333 ton design and were studying a 35,000 ton design.
The French decided to go forward with 23,333 tons as a compromise between an inadequately armoured 17,000 ton ship and a prohibitively expensive 35,000 ton ship. The design was 213m long with a 27.5m beam, armed with two quadruple 305mm/55 calibre turrets forward and three quad 130mm dual purpose turrets aft, had a 230mm armoured belt and 150mm horizontal protection, and a speed of 30 knots. The design was submitted to Parliament in May and rejected after two months of debate; it seemed excessive to counter the (officially) 10,000 ton Deutschland with a ship twice the size, and why the 35,000 ton design was not preferable. In July, the navy allowed to carry out revisions for resubmission later.
Vice Admiral Durand-Viel became Chief of Staff in January 1931 and ordered a study replacing the 305mm/55 guns with 330 mm/50 guns to outmatch the old Italian battleships, which carried 320mm/45 guns. This increased displacement by 3000 tons to 26,500 tons, length by 2 metres, beam by 2,5 metres, and reduced speed to 29.5 knots. Two more double 130mm DP turrets were added, and the thickness of the armoured belt and decks were increased slightly. This design was approved by Parliamentary committees in early 1932, and Dunkerque was ordered on October 26, and laid down on December 24.
The German response to Dunkerque was the Scharnhorst class of battleships, both laid down in 1935. They were larger, had better armour, but nine guns of only 280 mm caliber. These guns were improved versions of the type used by the Deutschland-class with longer barrels and higher muzzle velocity. A heavier armament had been pre-empted by negotiations for the 1935 Anglo-German Naval Agreement. As the Dunkerques were designed to withstand 280mm shells, the French saw no need for a new class in reply.
Rather, it was the Italians who provoked the next class of French capital ship. On 26 May 1934, Duce Benito Mussolini announced Italy would exercise its treaty right to build new battleships; this was confirmed a few days later when Agenzia Stefani reported the laying down two 35,000 ton Littorio-class battleships. The immediate French response was to build a second Dunkerque with increased vertical armour. Strasbourg was ordered on July 16 and laid down in November.
The main differences with Dunkerque were the thickness of the belt armor amidships (283 mm (11.1 in) in place of 225 mm (8.9 in)), of the transverse bulkheads, and the vertical armor of the main battery turrets, inducing an 750 t (740 long tons) increase of weight, and the bridge installation incorporating the conning tower in two tiers, in the fore tower.
Strasbourg was ordered on 16 July 1934 in response to the Italian Littorio-class battleships. The ship was laid down on the N°1 slipway of the civilian Penhoët Shipbuilding Yards, at Saint-Nazaire, which had been built to accommodate the 313-meter long keel of the liner SS Normandie. She was launched in December 1936, to be fitted out, and left Saint-Nazaire for Brest on March 15, 1938 for her acceptance trials which were carried out, Strasbourg entered service in April 1939, joining the French Atlantic Fleet, and forming with Dunkerque the 1ère Division de Ligne (1st D.L.). White bands were painted on the funnel, in March 1939, a single one on Dunkerque as Division flagship, two on Strasbourg.
After an official visit on 3–4 May, to Lisbon (Portugal), for the commemoration of the discovery of Brazil by Alvares Cabral, both battleships, accompanied by three modern light cruisers of the 7,600-ton type, forming the 4th Cruiser division, visited in late May and early June, Scottish harbours and Royal Navy bases, Liverpool, Oban, Staffa, Loch Ewe, Scapa Flow, and Rosyth, returning to Brest after a four-day call at Le Havre.
In the first days of September 1939, the Force de Raid, under Vice Amiral d'Escadre (Squadron Vice Admiral) Gensoul on Dunkerque, including the 1st D.L., the 4th Cruiser division, and eight large destroyers was based in Brest. It sortied immediately, as German pocket battleships were reported seen, wrongly, to stop them from passing into the Atlantic. It was soon decided to split the Force de Raid into hunting groups against the German surface raiders, incorporating Royal Navy warships.
In October–November 1939, Force X, under Vice Admiral Duplat on the French heavy cruiser Algérie with Strasbourg, French heavy cruiser Dupleix and the aircraft carrier HMS Hermes, was based at Dakar and vainly undertook sweeps trying to find Admiral Graf Spee. When Strasbourg returned to Brest, 800 of her powder charges remained in storage at Dakar. During the battle of Dakar, this powder was used by Richelieu and was (wrongly) implicated in the explosion of Richelieu's upper turret 380 mm gun barrels.
The Force de Raid was despatched, on April 2, 1940, to the Mediterranean to counter uncertain Italian intentions during the spring of 1940, but, some days later, was ordered to return to Brest to provide cover for an eventual Allies' reaction to the German landings in Norway, on April 9, 1940. Finally the Force de Raid was ordered to Mers-el Kebir on April 24, 1940.
The only test in battle for Dunkerque and Strasbourg came in the attack on Mers-el-Kébir, after the fall of France, from the Force H battleships, HMS Hood, HMS Resolution, and HMS Valiant though the French battleships had not been designed to confront these heavily armed battleships.
In the late afternoon on July 3, 1940, the surprise was terrific, as, the French crews could not believe, to the last moment, that the British warships were going to fire on them, and tactically, because their ships were tightly moored, guns trained forward, while the British warships were free in their movements, at 16,000 m, at high sea.
As the old super-dreadnought Bretagne, badly stricken in her magazines, blew up, capsized and sank, killing nearly 1,000 seamen, Dunkerque painfully breaking her mooring ropes, quickly suffered four 15-inch shell hits. The first shell bounced on the upper 330 mm turret roof, killing all the men in the right half of the turret as the left half turret remained operational, the second damaged the aircraft installations, the last ones, piercing the armored belt, damaged boilers and destroyed the electric power plant, thus the ship had to be moored on the other side of Mers-el-Kébir roadstead.
Strasbourg, on which the commanding officer, Captain Louis Edmond Collinet, had previously ordered to carefully prepare to cast off, was luckily near missed by the 15-inch (381 mm) shells of the British guns. At 18.00, a 15-inch salvo fell where her stern had been one minute previously. Escorted by five destroyers, she headed to the pass, then steered northeast. Strasbourg was then increasing speed from 15 knots to 28 knots but was hampered by damage to an air intake on the funnel, which had been blocked by a piece of flying masonry from the jetty. However she escaped the pursuit by Hood and Fairey Swordfish torpedo bombers of Ark Royal until about 21.30. At this moment, all thirty boiler room 2 personnel were found lying unconscious, overcome by the heat and the toxic fumes, and three petty officers and two seamen had died. Passing alongside the western coast of Sardinia, Strasbourg reached Toulon, in the evening of the following day.
With Bretagne lost, Dunkerque and Provence severely damaged, Lorraine interned at Alexandria, Courbet and Paris seized in Great Britain, only four heavy cruisers of seven and three light cruisers of twelve remained under Vichy control in Mediterranean waters, and with the Atlantic harbours under German occupation, a reorganisation of the Vichy French naval forces had to be carried out. The 1st (fast battleships) and 2nd (slow battleships) Divisions de ligne and the Atlantic Fleet were dissolved, in August 1940 and new Forces de Haute Mer (High Sea Forces) were created, with Admiral de Laborde appointed as C. in-C., on September 25, 1940, and raising his flag on Strasbourg, after the upper part of the fore tower had been fitted out to better accommodate an admiral and his staff. Vice Admiral Gensoul, though he received the fifth star of full Admiral, never got a further sea command.
Strasbourg, though flagship of so-called High Seas Forces, nearly never went to sea, due to the stringencies of fuel supply, but to cover, in November 1940, with two heavy cruisers, two light cruisers and some destroyers, the return to Toulon of Provence, escorted by five new Hardi-class destroyers, and abundant air cover. The two forces joined up off the Balearic Islands, and reached Toulon, on November, 8.
She received, in 1941, three more single 13.2 mm Browning CAS MG, and was fitted in 1942, with a so-called détecteur électro-magnétique, French ancestor of a sea and air warning radar. Four small rectangular antennæ were fitted atop the main yards of the fore tower, the starboard fore and the port after antennæ were for transmission, the other pair for reception, a different arrangement of the one fitted on Richelieu at Dakar, in 1941. Early tests indicated a range against aircraft of 50 km.
Strasbourg was still at her moorings of the Milhaud finger piers at Toulon when the Germans invaded the so-called "Zone libre", in retaliation of the Allies' landings in French North Africa. On November 27, 1942, when the Germans attempted to seize the French warships remaining under Vichy control, she was scuttled by her French crew on November 27, 1942 as part of a pre-planned effort to keep the ships from being turned over to the Italian Navy. She was refloated July 17, 1943 by the Italians, but the armistice between Italy and the Allies in September 1943 halted these activities and the ship was taken over by the Germans. On April 1, 1944 they handed her back to the Vichy French authorities. Her wreck was then towed to the Bay of Lazaret, where she was heavily bombed by the US aircraft, and sunk, three days after the August 15, 1944 landing, as part of the preparations for liberation of Toulon. She was raised for the second time on 1 October 1944 but found to be beyond repair, and used as a testbed for underwater explosions until condemned and renamed Q45 on 22 March 1955, to be sold for scrapping on 27 May that year.