The ship was built in Bordeaux, France, for the Confederate States Navy under the cover name Sphinx, but was sold to Denmark after the sale of warships by French builders to the Confederacy was forbidden in 1863.

The Tokugawa shogunate of Japan bought her from the United States in 1867 and renamed her Kōtetsu, but delivery was held up by the Americans until after the Imperial faction had established control over most of the country.

She was finally delivered in March 1869 to the new government and had a decisive role in the Naval Battle of Hakodate Bay in May, which marked the end of the Boshin War, and the completion of the military phase of the Meiji Restoration.

[Note 1] The brig's composite hull was sheathed in copper to protect it from parasites and biofouling and it featured a pronounced tumblehome.

[3] Her main battery consisted of a single 300-pounder 10-inch (254 mm) Armstrong rifled muzzle-loading (RML) gun located in the bow turret in a pivot mount.

Her hull was protected by a wrought-iron armored belt that extended 2.12 meters (6 ft 11 in) below the waterline that was backed by about 15 inches of teak.

[8] The following month Bulloch entered a contract with Lucien Arman, a French shipbuilder and a personal confidant of Napoleon III, to build a pair of ironclad rams capable of breaking the Union blockade.

[9] Prior to delivery, however, a shipyard clerk walked into the U.S. Minister's office in Paris and produced documents which revealed that Arman had fraudulently obtained authorization to arm the ships and was in contact with Confederate agents.

[10] The French government blocked the sale under pressure from the United States, but Arman was able to sell the ships to Denmark and Prussia, which were then fighting on opposite sides of the Second Schleswig War.

Heavy weather forced the ship to take refuge at Elsinore, but she set sail shortly afterward for the French coast.

High seas in the Bay of Biscay damaged her rudders while en route for the island of Madeira, Portugal, and forced the ship to seek refuge in Ferrol, Spain.

Stonewall reached Nassau, Bahamas, on 6 May and then sailed on to Havana, Cuba, where Page learned of the war's end when he arrived five days later.

While sailing through Chesapeake Bay on the night of 22/23 November, Stonewall accidentally rammed and sank a coal schooner off Smith Island, Maryland; there were no deaths.

On 25 March 1869, during the Battle of Miyako Bay, Kōtetsu successfully repulsed a surprise night attempt at naval boarding by the rebel Kaiten (spearheaded by survivors from the Shinsengumi), making use of a mounted Gatling gun.

[19][21][22][20] Following the end of the Boshin War in August 1870, Kōtetsu was classified as a third-class warship on 15 November 1871 and was renamed Azuma on 7 December.

[19] Her armor plating was reused to make the armature shafts in the electric generators in the Asakusa Thermal Power Station, built in Tokyo in 1895.