The Dominion's capture of Betazed during the Dominion War, a crushing blow that pushed Benjamin Sisko to bring the Romulans into the war by deception.
The Battle of Betazed was the Dominion's seizure of the planet Betazed, homeworld of the Betazoids and a core member of the United Federation of Planets. The Dominion fleet overwhelmed the planet's defences and occupied it, giving the enemy a foothold uncomfortably close to the heart of Federation space.
The fall of Betazed is reported in the episode In the Pale Moonlight, set in 2374. It comes after a run of losses in the war and alarms Starfleet, since the Federation lacks the strength to defeat the Dominion alone. Motivated by the conquest, Benjamin Sisko agrees to Elim Garak's plan to fake evidence of a Dominion plot against the Romulan Star Empire. When the forgery is exposed, Garak arranges the death of the Romulan senator Vreenak, and the Romulans enter the war on the Federation's side.
The writers considered having Vulcan fall but chose Betazed as an important Federation world that would still shock viewers. The battle highlights how desperate the war had become and frames Sisko's darkest moral compromise.