On 12 October 1861 , the CSS Manassas became the first ironclad to enter combat , when she fought Union warships on the Mississippi during the Battle of the Head of Passes . She had been converted from a commercial vessel in New Orleans for river and coastal fighting . In February 1862 , the larger CSS Virginia joined the Confederate Navy , having been rebuilt at Norfolk . Constructed on the hull of USS Merrimack , Virginia originally was a conventional warship made of wood , but she was converted into an iron @-@ covered casemate ironclad gunship , when she entered the Confederate navy . By this time , the Union had completed seven ironclad gunboats of the City class , and was about to complete the USS Monitor , an innovative design proposed by the Swedish inventor John Ericsson . The Union was also building a large armored frigate , the USS New Ironsides , and the smaller USS Galena .