The Starfleet 12th Fleet was a military unit, a fleet group of the Federation Starfleet during the Dominion War. In early 2375, the Twelfth Fleet was assembled at Starbase 19, in preparation to drive Dominion occupying forces from the planet Betazed. However, thirty-six ships comprising the fleet were destroyed in a pre-emptive attack by the Dominion. The USS Nautilus was one of the ships in the Twelfth Fleet. (TNG novel: The Battle of Betazed)