The Battle of the Omarion Nebula was the first major engagement between Alpha Quadrant forces and the forces of the Dominion; it was also the only major engagement to take place in the Gamma Quadrant. It was a major defeat for Alpha Quadrant forces, likened to the Federation's defeat by the Borg at the Battle of Wolf 359.
During 2371, various factions of Alpha Quadrant powers became increasingly worried about the threat posed by the Dominion and its shape-shifting Founders. Two groups, the Romulan Tal Shiar and the Cardassian Obsidian Order, formed a secret alliance to expedite the destruction of the Dominion. This alliance, led by former Obsidian Order chief Enabran Tain, gathered enough material and manpower to construct a fleet of 20 Romulan D'Deridex-class Warbirds and Cardassian Keldon-class heavy cruisers in the Orias system. (DS9: "Defiant", "Improbable Cause")
Late that year, the fleet was launched. It entered the Bajoran wormhole, and upon exiting set course for the Founder's Homeworld in the Omarion Nebula. The fleet believed it would remain hidden from the Jem'Hadar through their improved cloaking devices, thereby preventing the Founders from evacuating or sending for reinforcements before the fleet reached their planet. The fleet then planned to bombard their homeworld from orbit for six hours until the planet had been totally destroyed.
The plan was a complete disaster, because unbeknownst to Tain and the rest of the fleet, the chief Romulan officer, Colonel Lovok, was actually a Changeling. The Founders had completely evacuated the planet beforehand, and had ordered a fleet of 150 Jem'Hadar fighters to lie in wait within the surrounding nebula, ready to obliterate the enemy fleet. The Founders had actually helped the plan along in the belief that obliterating the Tal Shiar and the Obsidian Order would cripple the Romulans and the Cardassians. The Founders left an automated transponder to send back false sensor readings on their homeworld's surface so that the fleet would not suspect anything was amiss until the attack began.
The Founders' plan worked perfectly. The combined Romulan/Cardassian fleet destroyed 30 percent of the planet's crust before realizing that it was deserted. By this point, it was too late, and the amassed Jem'Hadar fleet emerged from the nebula and attacked. The Romulan/Cardassian fleet was obliterated, and the few survivors were interned in several Dominion interment camps, including Enabran Tain himself. Only Elim Garak escaped with the help of the USS Defiant, which was on a mission to rescue Odo from the Romulan/Cardassian fleet. (DS9: "The Die Is Cast", "In Purgatory's Shadow")