The Concordia suffered several torpedo hits defending Earth (in the novel Fleet Action). After the Battle of Terra she was put on "indefinite patrol status, Code 4" and spent some eight months patroling 'safe' sectors. She was destroyed by the Kilrathi in the Vespus System (an important fleet base), fighting a rearguard action. That's all we really know -- presumably she was covering a Confederation retreat (rearguard is the opposite of vanguard - the part of a force that is in back to cover a retreat), but no details are available.