During the American Civil War, what is now The United States of America was divided into the Confederacy (the South) and the Union States (the North). The North and the South clashed; the South wanted to keep its independence as the Confederate States of America, and the North wanted to regain the lost states.
During this time, the Confederate and Union forces had a heavy interest in conquering the seas. The Union wanted to blockade the South's imports and exports, and the Confederates wanted to avoid that. There were many battles at sea before, but before The Monitor and The Merrimack (sometimes spelled 'Merrimac') there were only wooden ship battles. So the battle between these ships changed the war and the way ships were made from then on.
Before The Monitor and The Merrimack
Although there had never been a battle between two armoured ships, The Monitor and The Merrimack were by no means the first of their kind. The US Navy had debuted Ol' Ironsides before these, and, predating that, in 1592, Korean Admiral Yi-sun made the first iron ship to repel the Japanese Military. There also were ironclad construction programmes under way in France and in the UK.
Originally, The Merrimack had been a Union ship, which was burned and sunk by Federal forces in the Norfolk Navy Yard in 1861 to make it unusable for the Confederacy. However, it was raised and was converted into an ironclad warship now named Virginia. Unofficially, it was still referred to by its Union name.
The First Battle
The day before the battle, the 20-gun Merrimack had destroyed two Union ships, the USS Cumberland and the USS Congress and eventually was engaged by the small Union two-gun USS Monitor. Ordinarily, a small ship like the Monitor wouldn't attack the Merrimack, but it was defending another beached Union ship, the wounded USS Minnesota.
This marked the first battle between two ironclad ships.
The two Union named ships engaged in a four-and-a-half hour battle on 9 March, 1862, with almost no structural damage sustained by either ship. This marked a turning point in naval warfare. Captain Buchanan commanded The Merrimack, and Lieutenant Worden commanded The Monitor. The Merrimack had sunk only wooden ships before and was unable to destroy The Monitor. Though The Merrimack had 18 more guns, The Monitor fought it into a draw. The Merrimack had sustained almost no damage from the previous two ships, and was in good shape, which is one of the biggest surprises of this battle.
Neither side could claim victory since The Merrimack retreated and The Monitor had 16 times the deaths of its adversary.
In April, the Confederate ship once again advanced The Monitor, this time led by Captain Tattnall, but the Union ship denied any combat. Later on, The Merrimack was dismantled due to a Confederate General's inability to set her on fire. The Monitor sank in heavy seas in 1862.
Today, the idea of using iron armour has moved onto land. The idea and design of tanks was largely affected by the American Civil War's new use of the ironclad ships. The success of iron cladding led to its being used as standard to armour military ships and vehicles.