The Battle of the Omarion Nebula was the first major armed engagement between 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.
The year 2371 was an anxious time for the various powers of the Alpha and Beta Quadrants, as the threat posed by the Dominion on the other side of the Bajoran wormhole loomed ever closer. The Jem'Hadar offensive of 2370, which had claimed amongst others the USS Odyssey, and the subsequent encounter between the USS Defiant and the Founders, had revealed a ruthless, technologically advanced enemy determined to impose their version of order on the chaotic galaxy. Although a full-scale invasion did not follow the initial appearance of the Jem'Hadar as was feared, war seemed to be only a matter of time.
Immediately the United Federation of Planets and the Klingon Empire began preparations for a Dominion incursion, including the fortification of Deep Space 9 and the investigation of methods to combat changeling infiltration. The Romulan Star Empire, ever cautious, loaned a cloaking device to the Federation (installed on the Defiant) in exchange for intelligence Starfleet gathered on the Dominion, but refused to send ships of their own to the Gamma Quadrant. The Cardassian Union, occupied by its own internal and external concerns, also remained relatively uninvolved.
At this time, the Cardassian Enabran Tain was living out his retirement on Arawath, as the only head of the Obsidian Order to survive long enough to do so. Frustrated by the Central Command's complacency in the face of the Dominion threat and wearied by his three-year absence from power, Tain drafted a plan to destroy the Dominion in a single decisive move. To accomplish this he contacted the Romulan Tal Shiar, and proposed a joint venture, executed independently of either organization's parent government. However, unbeknownst to him, his counterpart in the Tal Shiar, Colonel Lovok, had been replaced by a changeling. The Founders saw in Tain's plan their own opportunity, and did everything they could to carry it forward.
Through the Romulans, Tain had access to all the information gathered on the Dominion by Starfleet over the past several months. He learned of the Founders' control over the Jem'Hadar via their addiction to ketracel-white, and the location of their homeworld inside the Omarion Nebula. Tain concluded accurately that a massive attack directed towards the Founders' homeworld, eradicating the Founders and with them the supply of white, would cripple the Dominion irreparably.
In great secrecy, the Obsidian Order began building a fleet of Keldon-class warships in the Orias system, even though Cardassian law specifically prohibited the organization from operating military spacecraft of any kind. The fleet was equipped with cloaking devices provided by the Tal Shiar, which had been modified to avoid detection by Dominion scanners. Meanwhile, the Tal Shiar assembled its own force of several D'deridex-class warbirds. Numbering twenty ships altogether, their combined firepower was sufficient to obliterate the crust of the Founders' homeworld in one hour, and the mantle in five. Apart from those directly involved in the operation, the only individuals who knew of the Tain's plan were Elim Garak and Odo, who had been captured and held aboard Tain's flagship while investigating the deaths of several former Obsidian Order operatives.
Near the end of 2371, the combined Cardassian-Romulan fleet left the Orias system and headed for the Bajoran Wormhole. As they passed Deep Space 9, the ships decloaked and transmitted parallel messages to the Cardassian and Romulan governments. In the Cardassian message, Tain explained his motives as that of defending Cardassia, and advised the Cardassian fleet to be on alert for any Dominion reprisals. Both governments denied any knowledge of the mission, though despite the possibility of open war they elected to wait and see whether Tain would succeed.
Starfleet also chose not to interfere with the attack, admitting that Tain's plan had a fair chance of success. Vice Admiral Toddman ordered the civilian evacuation of Deep Space 9 and sent a task force of nine starships to reinforce the Bajor system. Meanwhile, Commander Benjamin Sisko disobeyed orders and took the Defiant into the Gamma Quadrant, in hopes of recovering Odo and Garak.
As the fleet entered orbit of the Founders' homeworld, they found the planet completely undefended, the Founders in a liquid state on the surface -- apparently they had achieved the element of surprise. Tain confidently ordered the fleet to decloak and open fire, with the words "so much for the Dominion." Thirty percent of the planet's crust was destroyed in the opening volley, but something was amiss: there was no change in the number of life-forms on the surface. They realized that the Founders had already evacuated the planet, leaving behind only an automated transponder to send out false sensor readings. The entire operation had been a trap.
Mere moments later, a massive fleet of 150 Jem'Hadar attack ships emerged from the nebula, completely surrounding Tain's fleet. The Jem'Hadar's phased polaron beam weapons tore through the shields of the Cardassian and Romulan ships just as they had the Odyssey less than a year before, inflicting heavy damage in a matter of minutes. Amidst the chaos, Lovok helped Odo and Garak escape, telling them that "no changeling had ever harmed another." The two fled Tain's warbird in the runabout USS Mekong just before it exploded, and were picked up shortly after by the Defiant.
Outgunned 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 out of the battle zone and escaped back to Deep Space 9. A few members of the fleet, including Tain, survived the battle and were detained at Internment Camp 371 until their rescue by Garak and Commander Worf in 2373.
By any measure the Battle of the Omarion Nebula was a catastrophe for the Romulan and Cardassian forces; the Tal Shiar suffered a severe setback from which it would not recover for years, and the Obsidian Order ceased to exist altogether. The Founders had identified both organizations as a definite threat due to their efficient and ruthless natures, and had exploited Tain's plan to wipe out both of them in a single stroke. Their victory had been so overwhelming that the Dominion had essentially eliminated two of the major powers in the Alpha/Beta Quadrants from challenging them in the short-term; after their loss neither the Cardassians nor the Romulans would be very willing to wage war against the Dominion.
The destruction of the Obsidian Order also had more far-reaching consequences, which in time the Founders would also exploit to their advantage. Without the constant presence of the Order ensuring loyalty amongst the population, the Cardassian resistance was finally able to topple the Central Command. This in turn led to the Klingon-Cardassian War and the Second Federation-Klingon War in 2372-2373, weakening the Alpha and Beta Quadrants as a whole and nearly destroying the Federation-Klingon alliance, the last remaining bulwark against the Dominion. In addition, the humiliation and damage suffered by the Cardassians would ultimately lead them to their alliance with the Dominion, sowing the seeds for the most devastating conflict in modern history, the Dominion War.