Completed in 1896, she was the sole member of her class, with a main battery of heavy guns mounted on the centerline and the first use of Belleville boilers.

As newer battleships were commissioned into the fleet, Brennus was relegated to the Reserve Squadron in the early 1900s and then served as a training ship.

Admiral Théophile Aube, who opposed battleship construction in favor of the cheaper torpedo boats and cruisers of the Jeune École, became the Naval Minister and cancelled both ships in January 1886 before much work had been done.

Aube left the government in 1887 and his successor permitted work to resume; the ship's designer, Charles Ernest Huin, lobbied to restart construction to keep the shipyards busy.

[3] The ship introduced several advances for the French fleet: new, 42-caliber guns arranged in turrets forward and aft of the superstructure, the first use of homogeneous steel armor, and the first Belleville boilers.

Other changes included the adoption of a thin upper strake of armor to protect against quick-firing guns and the abandonment of the ram bow.

[4] Brennus formed the basis for the subsequent group of five broadly similar battleships built to the same design specifications, begun with Charles Martel, though they reverted to the armament layout of the earlier Magentas which saw the main guns distributed in single turrets in a lozenge (quadrilateral) pattern.

Brennus proved to be grossly overweight; during her initial sea trials in 1893, her draft was 38 cm (15 in) greater than it should have been, and this was not in a fully loaded condition.

The ship's excessive topweight from her massive superstructure, heavy military masts and tall funnels contributed to the problem, together with the pronounced tumblehome of her hull.

The ship could carry 706 t (695 long tons) of coal, which gave her a range of 2,805 nautical miles (5,195 km; 3,228 mi) at a speed of 11 knots (20 km/h; 13 mph).

[11] Electricity for lighting, searchlights, pumps, ventilators and other equipment was provided by four 400-ampere dynamos, each driven by a dedicated internal-combustion engine.

[14] The ship's secondary armament consisted of ten 45-caliber Canon de 164.7 mm (6.5 in) Modèle 1893 guns, four of which were mounted in single turrets amidships; the other six were located directly underneath them in casemates.

The following month, the Arsenal de Brest submitted a proposal to lighten the ship which it estimated was 412 t (405 long tons) overweight, which was approved on 30 October.

The 340 mm turrets are too heavy and are not balanced, and when both are trained on the same beam the heel of the ship is such that the upper edge of the belt is level with the water.

Despite her problems, as the most modern capital ship of the French fleet, she served as the flagship of the Mediterranean Squadron under Vice-amiral (Vice admiral) Alfred Gervais.

During gunnery training exercises to test the new system, Brennus and the ironclad battleships Neptune and Marceau achieved 26 percent hits at a range of 3,000 to 4,000 m (3,300 to 4,400 yd).

After their completion he suggested a race between the battleships; after two hours Brennus trailed the newer Charles Martel and Carnot, but beat four older ships.

Navy Minister Édouard Lockroy then observed gunnery exercises aboard her in September that culminated with the sinking of the old floating battery Arrogante.

Vice-amiral Ernest François Fournier hoisted his flag aboard Brennus as commander of the Mediterranean Squadron on 1 October.

As tensions between France and Great Britain rose during the Fashoda Incident, the squadron's sailors had their leave canceled on 18 October and the battleships loaded their full complement of ammunition before disembarking it on 5 November when the two countries settled their differences.

[24] During the squadron's cruise of the Eastern Mediterranean in October–December 1899, the ship hosted a dinner for Queen Olga of Greece before returning to Toulon on 21 December.

The exercises began on 6 March, and Brennus went to sea with four of the battleships and four protected cruisers for maneuvers off Golfe-Juan, including night-firing training.

While cruising off Cape St. Vincent during the voyage back on the night of 10/11 August, Brennus accidentally collided with the destroyer Framée, sinking her and killing forty-seven of her crew; only fourteen men were rescued.

[29] After a short refit in January 1903, Brennus was one of the ships that visited Cartagena, in honor of King Alfonso XIII of Spain.

Navy Minister Gaston Thomson observed exercises aboard the battleship which visited Corsica and ports in southern France during 17–28 October.

There was a small fire on 12 June in the forward main-gun turret that badly burned the miscreant who had lit a gasoline-soaked rag.