An initial suggestion to fit the ships with 160-shaft-horsepower (120 kW) steam engines allowing for a speed of 4.5 knots (8.3 km/h; 5.2 mph) was declined as to minimise departures from Sané's design.

On 17 June 1852, the Ministry of the Navy suspended construction and required that the ships be lengthened by 3.43 metres (11 ft 3 in) and that 540 shp (400 kW) steam engines be incorporated.

Desaix, whose keel was only beginning to be laid, was cancelled altogether and Arcole, second ship of the Algésiras type, the production series of the Napoléon design, was started instead.

The figurehead figured the prophet Veleda, an important character in the folklore of Brittany, with a sickle in hand and a wearing an oak leaf crown.

Completed two years after her British homologue HMS Duke of Wellington, Bretagne became the most powerful warship in the world, but commissioned too late to effectively take part in the Crimean War, which was almost over after the fall of Kinburn in October 1855.

Appointed flagship of the Toulon squadron in January 1856, she sailed to the Black Sea to serve during the last months of the conflict, which came to an end in July, and helped return the French expeditionary corps back to France.

The visit was counter-productive, as the display of power of the French fleet, compounded by bouts of diplomatic clumsiness such as inaugurating an equestrian statue of Napoléon I, irritated and worried the British.

[1] After the British delegation departed in haste, Bretagne took the French emperor and empress aboard and ferried them to Brest for the next leg of their official tour.

Bretagne served as a troopship during the Second Italian War of Independence in 1859, and a few months later the led the bombing of Tétouan forts in Morocco, where a cannonball hit her hull.

She then sailed to Gaeta in October, under Admiral Adelbert Lebarbier de Tinan, to oppose a Sardinian attack against Napolitan forces, leading to the Battle of Garigliano.