Battle of Betazed
The Battle of Betazed was a surprise Dominion offensive against the Federation member world of Betazed during the Dominion War in 2374. It was an unmitigated disaster for the Federation and its allies, plunging the war into its darkest hour.

The battle
The Dominion assembled their forces in the Kalandra sector, which Starfleet Intelligence had believed was too far from their supply lines to present a major threat. The attack caught the Tenth Fleet, assigned to protect Betazed and its outlying colonies, out of position on a training exercise. The Dominion met little resistance from Betazed's obsolete planetary defense system, and was able to conquer the planet in a mere ten hours.

Beyond the immediate loss of a major Federation member planet, the battle had dire strategic implications for the Federation. With Betazed under its control, the Dominion was now in a position to strike at the heart of Federation territory, including such core worlds as Andor, Tellar, Alpha Centauri, and . ()

Aftermath
A few weeks later, the Dominion had solidified its hold on the Kalandra Sector and were trying to establish a supply line running through Betazoid space into the Argolis Cluster, which would allow them to launch an attack on . Starfleet planned to have the Seventh Fleet intercept them near the Tibor Nebula. ()

Later in 2374, the Second Fleet launched at least three attacks against the Dominion occupation forces, but were unable to make progress due to the Dominion continually reinforcing and fortifying their positions on the planet. ()

Appendices

Apocrypha 
In Tales of the Dominion War, the short story "The Ceremony of Innocence is Drowned" tells of what Lwaxana Troi was doing up to the battle and invasion, including mentioning the unfortunate yet heroic death of Mister Homn. 

The story is continued in The Battle of Betazed, a novel that tells of the fight to recapture Betazed from the Dominion in detail. Elias Vaughn, Deep Space 9's new first officer in the  relaunch novels, is instrumental in helping the  crew retake the planet.

bg:Битка за Бетазед
cs:Bitva o Betazed
de:Invasion von Betazed
ja:ベータゼットの戦い
nl:Slag om Betazed
Betazed, Battle of