Before conversion into a Confederate ironclad, CSS Virginia was the Union steam frigate USS Merrimack.
The United States Navy commissioned Merrimack in 1856. It was named for the Merrimack River, which flows through New Hampshire and Massachusetts. After Virginia seceded in 1861, Confederate forces captured the Gosport Navy Yard near Norfolk, where Merrimack had been partially burned and abandoned by the Union.
The Confederacy raised the vessel and rebuilt its surviving hull as an ironclad. Workers fitted it with heavy armor made from railroad iron and equipped it with powerful guns. The converted ship was renamed Virginia, although Union observers and many later writers continued to call it Merrimack or Merrimac.
Virginia fought the USS Monitor at the Battle of Hampton Roads in March 1862, the first famous clash between ironclad warships. The battle ended without a clear tactical victor, but it demonstrated that armored steam-powered ships had changed naval warfare. A common mix-up is “Merrimac”: that spelling was historically used, but Merrimack was the ship’s official Union name.