Answer:
The Monitor and the Merrimack
Step-by-step explanation:
The Merrimack (aka the Virginia) was the Confederate ironclad that threatened the Union's naval blockade. The Merrimack (as it was originally named) had been a conventional steam frigate. The South had salvaged the ship and renamed it the Virginia. The upper hull was cut away and then covered in iron armor.
The Monitor was the Union's ironclad ship built in response to the Confederate's ironclad.
The two fought each other at the Battle of Hampton Roads, (March 9, 1862), in the harbor at the mouth of the James River. The outcome of battle between the two ironclads was not decisive for either side -- but did issue in a new era in naval warfare.