Colonel Lovok, in command of the joint Cardassian-Romulan fleet
The Cardassian-Romulan fleet fires weapons at the Founders' homeworld
The Battle of the Omarion Nebula was the first major armed engagement between the forces of the Alpha and Beta Quadrants and the Dominion, as well as the only major engagement to take place in the Gamma Quadrant. In the battle, a combined Obsidian Order and Tal Shiar fleet attempted to launch a preemptive strike on the Founders' homeworld in the Omarion Nebula, only to be ambushed and completely destroyed by the Jem'Hadar. This disastrous defeat has been likened to the Federation's defeat by the Borg at the Battle of Wolf 359.
With the Dominion secretly gaining a foothold in the Alpha Quadrant, the former leader of the Obsidian Order, Enabran Tain, grew frustrated by the Cardassian Central Command's complacency in dealing with the Dominion threat. Tain drafted a plan to destroy the Dominion in one engagement. To accomplish this, he contacted the Romulan Tal Shiar to discuss a joint venture. Unfortunately, Tain's counterpart in the Tal Shiar had been replaced by a Changeling. The Founders working through Lovok did everything in their power to make sure the plan proceeded.
In great secrecy, the Obsidian Order (who was prohibited from engaging in military operations) began to assemble a fleet of Keldon-class warships in the Orias system. The Cardassian fleet was fitted with cloaking devices modified to avoid detection by the Dominion. Meanwhile, the Tal Shiar assembled its own force of several D'deridex-class warbirds. Numbering twenty ships altogether, their combine firepower was sufficient to obliterate the crust of the Founders' homeworld in one hour, and the mantle in five.
Near the end of 2371, the combined Cardassian-Romulan fleet left the Orias system and headed for the Bajoran Wormhole. They passed Deep Space Nine to transmit a message to their respective governments. Starfleet chose not to interfere in the attack, anxiously awaiting word of the results. Furthermore, Vice Admiral Toddman ordered a task force of nine starships to reinforce the Bajoran system.
As the fleet entered orbit of the Founders' homeworld, they found it completely undefended. Apparently achieving surprise, Enabran Tain confidently order the fleet to decloak and open fire. Their joy turned to dread as the information showed no change in the number of lifeforms on the surface. They soon realized that the Founders were not there. The operation was a trap.
Moments later, a Dominion fleet of 150 Jem'Hadar attack ships emerged from the nebula, completely surrounding Tain's fleet. The Jem'Hadar's phased polaron beam weapons ripped the shields of the Cardassian and Romulan ships, inflicting devastating damage in a short time. Amidst the chaos, Colonel Lovok helped Odo and Garak escape, confessing to Odo that "no Changeling has ever harmed another." The two fled Tain's warbird in the runabout USS Mekong just before the warbird exploded. They were picked up a short time later by the USS Defiant.
Outgunned and with no hope of retreat, the remnants of the Cardassian-Romulan fleet fought to their annihilation. As the Jem'Hadar ships continued to focus on the fleet, the Defiant fought its way back to Deep Space Nine.
The destruction of the fleet sent ripples through the Cardassian and Romulan governments. The Obsidian Order ceased to exist altogether and the Tal Shiar suffered a sever setback from which it would not recover for some time. The Dominion had labeled both organizations as threat and their plan to cripple them worked to perfection.