The monitor and her escorts departed in late 1865 and reached the Chilean port of Valparaíso in early 1866 where the Americans unsuccessfully attempted to prevent the Spanish from bombarding the undefended town during the Chincha Islands War.

[4] Her main battery consisted of four smoothbore, muzzle-loading, 15-inch (381 mm) Dahlgren guns mounted in two twin-gun turrets, one each fore and aft of the single funnel.

[8] Although the Navy had believed that its fire was accurate and effective,[7] it was neither because many gunners had aimed at the Confederate flag flying above the fort and their shells had flown across the peninsula to land in the Cape Fear River.

[9] A second assault was begun on the morning of 13 January 1865 with the ironclads the first to fire in the hopes of provoking the Confederate gunners to retaliate and reveal the positions of their gun so that they could be engaged by the rest of the fleet.

[10] After Rear Admiral David D. Porter ordered that his ships were to aim at the walls of Fort Fisher rather than the flag, the bombardment was much more effective and many guns were dismounted or disabled.

Returning to Hampton Roads on 7 April,[6] Monadnock was assigned to the squadron commanded by Acting Rear Admiral Sylvanus Godon, which had been established to search for the Stonewall.

[17] The squadron departed two days later and put into Charleston Harbor on the 22nd to re-coal and to be reinforced by the monitor Canonicus before continuing on to Havana, Cuba.

[18] Monadnock's presence no longer required, she arrived at Norfolk, Virginia, on 12 June and continued onwards to the League Island Navy Yard in Philadelphia, Pennsylvania, to prepare for her impending voyage to California.

[6] To prepare the monitor for the voyage, she was fitted with a 3-foot-6-inch (1.07 m) breakwater to prevent head seas from battering her forward turret and tall, wooden pilot houses above the existing ones.

Stokers collapsed daily from heat prostration and special inducements of extra pay and spirits had to be offered for men to take their place.

Commodore John Rogers, commander of the American squadron, attempted to persuade Admiral Casto Méndez Núñez to forego the bombardment, but the latter claimed it was a point of Spanish honor.

Rogers even had his ships clear for action in an unsuccessful attempt to intimidate Méndez Núñez and was prepared to open fire if he received support from the small British squadron in the harbor.