With the outbreak of war between the states in the American Civil War (1861-1865), Union warplanners moved quickly to strengthen their position on inland waterways and commissioned many vessels for the task. USS Onondaga was ordered on May 26th, 1862 for this purpose and was designed as a river monitor - a large river-going warship featuring heavy guns and shallow draughts. Onondaga, described as a double-turreted river monitor, was laid down (by Continental Iron Works) that same year and launched on July 29th, 1863. She was commissioned for service on March 24th, 1864 and survived the whole of the war in service with the North. Decommissioned on June 8th, 1865, she was sold off to France where, rather interestingly, she continued under the same name. Her hulk was sold for scrap in 1904.
As built, USS Onondaga displaced 2,635 tons (short) and held a length of 226 feet with a beam of 51.5 feet and draught of 12.9 feet. Her power came from 4 x boilers feeding 2 x Horizontal back-acting steam engines driving 2 x propellers. The crew complement numbered 130 and armament centered on 2 x 15" Dahlgren smoothbore guns as a primary battery. The secondary battery was made up of 2 x 150-pounder Parrott rifled guns. The ship's profile involved a continuous flat main deck with a forward and afterward turret emplacement straddling a central smoke funnel. Armor protection ranged from 11.75" at the turrets to 5.5" at the waterline and 1" at the deck.
USS Onondaga's notable action of the war was as part of the James River Flotilla, participating in General Grant's march on Richmond, the Confederate capital. From there came a forgettable action against a Confederate river squadron which resulted in Onondaga's commanding officer facing court martial for "neglect of duty" following the decision to move the vessel further downstream. In the months after the war ended, the warship traveled back north where she ultimately faced decommissioning in New York waters.
In French service, she was given 9.4" rifled main guns, these supplanting the existing gun batteries of the original design. She was acquired on March 7th, 1867 and commissioned into the French Navy on June 15th, 1869, operating until the end of the century before being given up.