The Second Battle of Deep Space 9 was the opening battle of the Dominion War, in which the combined forces of the Dominion and the Cardassian Union, led by Gul Dukat, launched an assault on the Federation-held starbase Deep Space 9. Although the station was captured by the Dominion, the battle was a strategic victory for the Federation because they successfully mined the Bajoran wormhole, preventing the Dominion from sending reinforcements to join the battle from the Gamma Quadrant. Also, the commitment of forces necessary to attack the station enabled a joint Federation-Klingon task force to destroy vital Dominion shipyards.

Following their annexation of the Cardassian Union around stardate 50560, the Dominion began fortifying their foothold in the Alpha Quadrant with weekly convoys of warships and troops through the wormhole. As time went on, Cardassian space became a veritable fortress, teeming with Jem'Hadar soldiers and their Vorta overseers.

Although a technical state of war did not exist, and neither side had fully committed to immediate combat, numerous skirmishes took place along the border during this period. Aside from the Dominion's attempt to destroy the Bajoran system using a trilithium bomb, the Jem'Hadar and the Cardassians attacked and destroyed numerous Federation and Klingon patrol ships in a sustained campaign across the border sectors. Casualties of these attacks included the USS Tian An Men and the IKS B'Moth.

Gowron attempts to persuade Worf to join his attack on Cardassia. (DS9: "The Way of the Warrior")