At the outbreak of the Civil War, she was trapped in port at New Orleans, Louisiana, and was seized for use as a Confederate blockade runner as the "CSS Tennessee" in 1861, although she was never able to escape blockade of the New Orleans harbor. After the Union capture of New Orleans, the ship was put into armed Union service, including as the flagship of United States Navy Admiral David G. Farragut for the conclusion of the Mississippi Campaign. As USS "Tennessee", she was not only a fast and effective blockade ship in the West Gulf Squadron, but also a powerful gunship used to bombard Ft. Morgan during the Battle of Mobile Bay.