against the Confederate James River Squadron. When the Confederate squadron steamed down river to attack the weakened Union forces, Onondaga dropped downstream to a position where she could maneuver more easily. She and her supporting gunboats awaited them only to see the attack aborted when elements of the Confederate fleet—the casemate ironclads Virginia II and Richmond, the gunboat Drewry, and the torpedo boat Scorpion—all ran aground as the tide ebbed trying to pass Union obstructions at Trent's Reach, after the casemate ironclad Fredericksburg and the gunboat Hampton had passed through the obstruction at 01:30. The latter two ships continued downriver and anchored below the Dutch Gap Canal to await the rest of the squadron. The squadron commander, Flag Officer John K. Mitchell, recalled them several hours later and ordered them to anchor further upstream under the guns of Battery Dantzler. As the Confederate ships were refloated at 10:45, Onondaga approached the Union obstructions and opened fire at an estimated range of . Her Dahlgren guns hit Virginia II twice with solid shot; the first shot penetrated her armor and badly damaged the wooden backing and its supporting beams. The second shot killed one crewman and killed two others as it penetrated the armor and its backing. Richmond was only hit by a glancing shot that damaged the armored shutter that protected the ship's stern gun port before the Confederate ships were able to withdraw upriver. The decision to move Onondaga downstream resulted in her commanding officer, Commander William Parker, being court-martialled for "neglect of duty in not offering battle to the Confederate ironclads" during this engagement. Secretary Welles set aside the guilty verdict on a technicality. Onondaga continued to support Union troops for the rest of the war and steamed north after its end on 9 April. The ship was decommissioned at New York City on 8 June and was laid up at League Island, Pennsylvania. By an Act of Congress approved on 7 March 1867, the monitor was sold to back to her builder who resold her to France for 4,330,599 francs. French service Onondaga retained her name in French service and was towed from New York City to Halifax beginning on 2 September 1867. She was towed across the Atlantic to Brest by the transport departing on 15 June 1868, escorted by the aviso . The ship began an extensive refit after her arrival on 2 July that