Wooden-hulled vessels plated with 4.3 inches (109 mm) of wrought iron, they were armed with a battery of twenty guns in a broadside arrangement.

Formidabile had been damaged by Austrian coastal fortifications the day before, and had withdrawn for repairs; Terribile was ready for action, but had been preparing to attack Lissa and was too far south to take an active role in the battle.

[1][2] These ships were the first components of a major naval expansion program that was designed to prepare a fleet of ironclad warships capable of defeating the Austrian Navy.

[5] The first ironclads to enter service in the Regia Marina of the newly-unified Kingdom of Italy, Formidabile and Terribile served as the core of the fleet that was later fleshed out by the Principe di Carignano, Re d'Italia, and Regina Maria Pia classes that would see action during the Third Italian War of Independence against the Austrian Navy.

[7] After initially remaining in port, the Italian fleet under Admiral Carlo Pellion di Persano launched an attack on the island of Lissa in mid-July; the Austrian fleet under Rear Admiral Wilhelm von Tegetthoff sortied to mount a counterattack, which resulted in the Battle of Lissa on 20 July.

Formidable, which had been badly damaged during an engagement with Austrian coastal fortifications on Lissa the day before, had withdrawn by the time Tegetthoff arrived and took no part in the battle.

Terribile also played no role in the action, as she had been stationed too far to the south in preparation for another attack on the island, and she arrived on the scene of the battle only after the two fleets had disengaged.

Neither ship played a role in the attack on Civitavecchia in 1870—the last stage of the Italian wars of unification—owing to the very poor state of the Regia Marina in the aftermath of Lissa.