Ironclad warships, frequently shortened to just ironclads, were ships sheathed with thick iron plates for protection. The first uses of iron for naval protection occurred in the Far East in the 16th century, but the heyday of the ironclad came in the mid-19th century, when ironclads supplanted wooden ships, and then were themselves replaced by ships made of steel.

Premodern ironclads

Oda Nobunaga, a Japanese daimyo, had six iron-covered Ōatakebune (大安宅船) made in 1576. These ships were called "Tekkousen" - literally, iron armored ships - and were armed with multiple cannons and large caliber rifles to defeat the large, but not iron-covered, vessels enemy used. He defeated Mori's navy with them at the mouth of the Kizu River, Osaka in 1578 in a successful naval blockade. They are regarded as floating fortresses rather than warships and were never used in the open sea.

The Koreans developed Geobukseon ("Turtle[-shaped] ships") in the 16th century to thwart the repeated attempts by Japan to invade Joseon. The geobukseon—designed by the admiral Yi Sun-sin—were said to be ironclads; however, they were not fully covered but just roofed with iron plates or metal thorns so that enemy soldiers could not take the ships.

Modern ironclads

The French La Gloire, the first ocean-going ironclad warship.

In 1855, the French navy experimented with ironclad floating batteries as a means of reducing the fearsome Russian defenses at Sevastopol. In 1856, the British constructed two similar devices to reduce Russian coastal defenses in Lithuania, but failed to use them before the conclusion of hostilities. While these devices were ironclad, and built on floating "rafts", they had to be towed into position due to inadequate motive power of their own.

In 1859, France launched La Gloire, the first ocean-going ironclad warship in history. She was soon followed by the British Navy's much superior Warrior in 1860.

The use of steam-powered ironclads in a conflict started in the American Civil War. The first of these vessels to see action, CSS Manassas, was a turtleback ironclad steam-tug formerly known as the Enoch Train. She was used in combat against the U.S. Navy and proved somewhat effective initially until U.S. ships learned to exploit her rather weak armor. The first engagement of two ironclad warships was the Battle of Hampton Roads, from March 8-9, 1862. Though the engagement was inconclusive, the Confederate ironclad CSS Virginia and her Federal counterpart, USS Monitor, became somewhat legendary, and helped to usher in a new age of armored, steam powered warships.

Spain used ironclads against Chile in 1864, but the largest battle involving ironclads of this type was the Battle of Lissa in 1866. Waged between the Austro-Hungarian and Italian navies, the battle pitted combined fleets of wooden ships of the line and ironclad warships on both sides in the largest European naval battle since the Battle of Trafalgar. The victory won by Austria-Hungary established it briefly as the predominant naval power in the Mediterranean.

The ironclad continued to be the dominant style of warship and developed into what is sometimes called the "old" battleship before being replaced by more advanced, far more seaworthy vessels known to history as pre-Dreadnoughts.

While the ironclad warship suffered from numerous flaws, the fact that it became the prominent naval weapon of its era and inspired nearly a century of progressively heavier armored warships can be ascribed to its massive advantage over the previous ships of the line in terms of protection. While a ship of the line could resist some damage, it was terribly vulnerable to fire and found itself completely outclassed by the new developments in naval armament beginning in the middle of the nineteenth century. Combined with the phenomenon of the steam engine, the ironclad warship could outfight, outgun, and eventually outrun even the most powerful three decker.

The age of the ironclad officially ended with the birth of the pre-dreadnought; however, its influence continued to be felt until the end of World War II, when naval theorists argued that the armored warship had outlived its usefulness. Recent naval encounters, however, have caused the concept of the armored warship to be re-evaluated, and perhaps the ironclad will live on in some form, after all.