Kōtetsu , later renamed Azuma , was the first ironclad warship of the Imperial Japanese Navy. She was designed as an armored ram for service in shallow waters, but also carried three guns. 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 Danes refused to accept the ship and sold her to the Confederates which commissioned her as CSS Stonewall in 1865. The ship did not reach Confederate waters before the end of the American Civil War in April and was turned over to the United States.

An ironclad was a steam-propelled warship protected by steel or iron armor constructed from 1859 to the early 1890s. The ironclad was developed as a result of the vulnerability of wooden warships to explosive or incendiary shells. The first ironclad battleship, Gloire, was launched by the French Navy in November 1859, narrowly preempting the British Royal Navy. However, Britain built the first completely iron-hulled warships.