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 Jem'Hadar had secretly assembled a large invasion force in the Kalandra sector, which Starfleet had believed was too far from Dominion territory to represent a major threat. At the same time, the Tenth Fleet, which was supposed to protect the planet and its outlying colonies, was caught out of position at the start of the invasion during a training exercise. The disadvantage was compounded by the relative weakness of Betazed's antiquated and undermanned defenses.

Ultimately, the Dominion conquered the planet in only ten hours – a humiliating defeat for the Federation.

On a strategic level, the defeat had even worse implications. With Betazed under its control, the Dominion was positioned within striking distance of the very heart of Federation territory, including such core member worlds as Andoria, Tellar, Alpha Centauri, and Vulcan.

Aftermath

A few weeks later the Dominion had solidified its hold on the Kalandra Sector. They were trying to establish a supply line running through Betazoid space into the Argolis Cluster, which would allow them to launch an attack on Vulcan. Starfleet planned to have the Seventh Fleet cut them off near the Tibor Nebula. (DS9: "The Reckoning")

Later that year, the Second Fleet launched at least three attacks against the Dominion positions on the planet, but the continual Dominion reinforcements and the fortification of the planet stopped them from succeeding.

This information taken from of Betazed Memory Alpha