Ironclads are steam-propelled warships of the later 19th century, protected by iron or steel armor plates. The ironclad was developed as a result of the vulnerability of wooden warships to explosive or incendiary shells. The first ironclad battleship, , was launched by the French Navy in 1859; she prompted the British Royal Navy to start building ironclads. After the first clashes of ironclads took place during the American Civil War, it became clear that the ironclad had replaced the unarmored line-of-battle ship as the most powerful warship afloat.