The Twelfth Fleet
was a numbered fleet in the United Federation of Planets Starfleet
, active during at least the 23rd
and 24th centuries
In 2373, the Twelfth Fleet was sent to assist the Twentieth Fleet in containing the Klingons in the Archanis sector. (Star Trek: Swiftfire: "Chariots of Korvat")
In 2377-78, the command base of the Twelfth was Starbase 343. (Ships of the Star Fleet, Vol. 4: Star Fleet Operating Forces. )
In the post-Dominion War period, the 12th was assigned to Sector 441. Its command base was Deep Space 12.
The Twelfth Fleet was also the nickname for the Starfleet International chapters based in Region 12 — states of the American Midwest. (Starfleet International: Scott A. Akers, et al., Master Genealogy: Starfleet Region 12)