Star Trek Expanded Universe

The Twenty-First Fleet was a Starfleet fleet that took part in the Dominion War.

In 2374, the Twenty-First was involved in fighting around the Bolarus system bottling the enemy up in the Kumarek system. It then took part in Operation Blue Room, which drove the Dominion forces back from the system. (The Dominion War Sourcebook: The Fires of Armageddon)