The ship was not complete by the time Germany won the Battle of France, and Jean Bart was rushed to Casablanca to escape advancing German troops.

While in Casablanca, the French attempted to prepare the ship for action as much as was possible in light of limited infrastructure and the necessary parts to complete the vessel.

Most work on the ship was completed by 1955, when she formally entered active service, and she conducted two overseas cruises to visit Denmark and the United States shortly thereafter.

When in 1934 Italy announced that it would begin building two Littorio-class battleships armed with 381 mm (15 in) guns, the French Navy immediately began preparations to counter them.

She was powered by four Parsons geared steam turbines and six oil-fired Sural water-tube boilers, which developed a total of 155,000 shaft horsepower (116,000 kW) and yielded a maximum speed of 32 knots (59 km/h; 37 mph).

The ship carried four Loire 130 seaplanes on the fantail, and the aircraft facilities consisted of a steam catapult and a crane to handle the floatplanes.

[5] She was launched on 6 March 1940, but following the initial German successes in the Battle of France in May, the builders realized that the normal process of fitting-out would have to be abandoned in favor of preparing the vessel to be moved from the port.

At this time, the ship's armament was mostly incomplete; the forward main battery turret had its guns and armor plate installed, but it lacked necessary shell handling equipment and fire control systems.

[6] While work was still being completed, the ship's commander, Capitaine de vaisseau (CV—Ship-of-the-line captain) Pierre-Jean Ronarc'h received orders on 11 June to depart for Casablanca in French Morocco as soon as was practical.

He estimated that 19 June would be the best opportunity based on the tides, and early that morning Jean Bart was towed out of the fitting-out dock by three tugboats at 03:30 and into a channel that had been quickly but not sufficiently dredged to allow her escape.

[b] The ship accidentally grounded in the darkness, bending one of her screw blades, but the tugs pulled her free and at 04:45, the crew got Jean Bart's engines started.

Jean Bart increased speed to 12 knots (22 km/h; 14 mph) and at 06:30, rendezvoused with the destroyers Le Hardi and Mameluk, which were to escort her to Casablanca.

While cruising off Lorient the next day, the destroyer Épée relieved Le Hardi and the three ships continued on at a speed of 24 knots (44 km/h; 28 mph).

In 1941, an improvised director was fabricated using a torpedo tube platform removed from the old destroyer Enseigne Gabolde as a base for a 14 m (46 ft) rangefinder.

Before the French vessels could mount a significant challenge to the invasion, Massachusetts and the heavy cruisers Tuscaloosa and Wichita, began bombarding the port while a group of SBD Dauntless dive bombers armed with 500 lb (230 kg) bombs from Ranger struck the ships in the harbor.

Jean Bart received a pair of bomb hits at 07:18; the first damaged the port catapult, started a fire, and caused the manual steering compartment to flood.

Massachusetts then engaged Jean Bart and at 07:25, scored a hit that penetrated both of the French ship's armor decks and exploded in the empty magazines for the missing 152 mm guns.

The next day, the ship's 90 mm guns were used to fire on advancing ground troops, and on 10 November, Jean Bart's forward turret engaged the heavy cruiser Augusta between 11:41 and 11:51, nearly hitting with her second salvo.

Three of her nine two-gun salvos straddled Augusta, forcing the cruiser to withdraw at high speed and prompting Ranger to launch a second air strike on the ship.

Admiral Raymond Fenard, the head of the French naval mission to the US, requested that the US Navy take the ship to the United States to be repaired and completed, as they had begun to do with Richelieu.

The first variant called for the quarterdeck to be modified to carry two large catapults capable of launching American TBF Avenger or British Fairey Barracuda torpedo bombers and F6F Hellcat or Supermarine Seafire fighters.

The second proposal was less ambitious; the aircraft facilities were abandoned in favor of seventeen twin 5 in turrets, while the 40 mm battery would be increased to twenty quadruple mounts.

[27][28][29] Options to complete the ship without US assistance were limited; the scuttling of the French fleet in Toulon in November 1942 wrecked much of the shipyard facilities, and Brest would not be liberated until September 1944.

Given the objections to the aircraft carrier conversion, the construction department drew up estimates for how long it would take to manufacture the necessary 380 mm gun barrels, rebuild the superfiring turret, and stockpile the necessary munitions for the guns; the resulting figures projected that it would take four years to complete the vessel and another year to manufacture the shells and charges.

[39] She was finally pronounced complete on 1 May 1955, and that month she embarked President René Coty for a visit to Denmark, escorted by the destroyer Surcouf.

After returning from the cruise, which also included a visit to Oslo, Norway, she left in July to represent France at a celebration in New York of the 175th anniversary of the landing of French soldiers led by the Comte de Rochambeau during the American Revolutionary War.

Jean Bart then joined the rest of the intervention force, which included the carriers Arromanches and La Fayette, Georges Leygues, the destroyers Surcouf, Kersaint, Cassard, and Bouvet, and an anti-submarine warfare group.

On 5 November, Jean Bart steamed south to bombard Port Said, but she fired only four rounds from her main battery before the landing was cancelled.

On 1 August, she was reduced to reserve status, despite the reluctance of many in the naval command to see the funds and effort expended in completing the ship be wasted.

Some consideration was given in 1964 to reactivate the vessel for use as a command ship during a series of nuclear experiments in the Pacific Ocean, but it was determined that the cost would have been too high, thus the cruiser De Grasse was used instead.