The Starfleet fleet responsible for guarding Betazed, which was away on a training exercise when the Dominion struck and took the planet within hours in 2374.
The Tenth Fleet is the Starfleet formation charged with protecting Betazed and its neighbouring systems during the Dominion War.
Its failure is reported in the Deep Space Nine episode In the Pale Moonlight. A Dominion force crosses from the Kalandra sector and seizes Betazed in a matter of hours. The planet's own defences were outdated and undermanned, and the Tenth Fleet had been caught out of position on a training exercise.
The loss places the Dominion within reach of Vulcan, Andor, Tellar and Alpha Centauri. For Captain Sisko it is the moment that convinces him the war cannot be won without the Romulans, and it drives him into his compromising partnership with Garak to bring them in.