Star Trek Expanded Universe

The Kalandra Offensive was the name of the Dominion assault in mid-2374 that saw the conquering of the Kalandra sector and the fall of Betazed. (The Dominion War Sourcebook: The Fires of Armageddon)

Following the set back with the loss of Deep Space 9 the Dominion regrouped and rebuilt its forces to continue its assault on the Federation and its Klingon allies. The Dominion increased its attacks on Federation and Klingon patrols, attacking them on a nearly daily basis. The attacks wore down Starfleet personnel and diminished their morale, particularly with successes as the complete destruction with all hands of the USS Cortéz. (DS9: "Far Beyond the Stars")

The Dominion's boldness included slipping across the Romulan Neutral Zone (which the Romulans turned a blind eye too) to ambush Federation starships. Several ships including the USS Cairo were destroyed in this fashion. (DS9: "In the Pale Moonlight")

The offensive began in earnest with the surprise invasion of Betazed. A stunning victory for the Dominion saw the destruction of the Starfleet Tenth Fleet and the capture of Betazed and its outlying colonies. (DS9: "In the Pale Moonlight"; The Dominion War Sourcebook: The Fires of Armageddon)

Eventually the entire Kalandra sector would fall to the Dominion, creating a crucial supply line between Cardassian space to Betazed allowing the Dominion to launch attacks against other Federation core worlds. (The Dominion War Sourcebook: The Fires of Armageddon)