Battle of the Omarion Nebula
The Battle of the Omarion Nebula was the first major armed engagement between forces of the Alpha Quadrant and the Dominion, as well as the only large-scale conflict to take place in the Gamma Quadrant. 

Acting on a plan originating with Enabran Tain, a combined fleet of twenty Cardassian and Romulan warships under the control of the Obsidian Order and the Tal Shiar launched a preemptive strike on the  in the Omarion Nebula. However, the effort had long been infiltrated by a , and shortly after attacking, the fleet was ambushed and destroyed by the Jem'Hadar, with few survivors.

Background 
The year 2371 was an anxious time for the powers of the Alpha Quadrant, as the threat posed by the Dominion on the far side of the Bajoran wormhole began to grow. The Jem'Hadar attacks in 2370, claiming the Bajoran colony of New Bajor and the Starfleet ship , and a subsequent engagement between the  and several Jem'Hadar fighters, revealed a ruthless, powerful empire determined to impose their version of order upon a chaotic galaxy. Although a full-scale invasion did not follow the initial first contact of the Jem'Hadar as was initially feared, war seemed to be inevitable. ()

Immediately, the Alpha Quadrant powers began to prepare for invasion. The United Federation of Planets assigned the Defiant to Deep Space 9 at the mouth of the wormhole to bolster its defensive capabilities. The Federation shared all of its intelligence with the Romulan Star Empire as per their treaty to allow a Romulan cloaking device to be installed on the Defiant. The Cardassian Union, occupied by its own internal and external concerns, remained relatively uninvolved. ()

Secret alliance 

At the time, ex-head of the Obsidian Order, Enabran Tain, was living out his retirement on Arawath. Frustrated by what he perceived to be the Central Command's complacency in the face of the Dominion threat, Tain drafted a risky venture to destroy the Dominion in a single decisive move; by wiping out the Founders. 

To accomplish this end, he enlisted the aid of the Romulan Tal Shiar, proposing a joint military venture into the Gamma Quadrant executed independently of either organization's parent government. However, unbeknownst to him, his opposite number in the Tal Shiar, Colonel Lovok, had been replaced by a Changeling infiltrator. The Dominion saw in Tain's plan a golden opportunity to destroy the two most effective security agencies in the Alpha Quadrant, and did everything they could to carry it forward. 

Through the Romulans, Tain had access to all the intelligence gathered by Starfleet on the Dominion. He reasoned, that as the Founders controlled the supply of ketracel-white, by eliminating the Founders, the Jem'Hadar would soon die, crippling the Dominion's capability to wage war indefinitely. 

In great secrecy, the Obsidian Order began assembling a fleet of fifteen  cruisers in the Orias system, although Cardassian law specifically forbade the Order from operating military spacecraft of any kind. The fleet was detected by forces of Central Command during an incident involving the Defiant and the Maquis, but the buildup continued unabated. The fleet had been equipped with cloaking devices provided by the Tal Shiar, which had been modified to avoid detection by Dominion sensors, provided they did not exceed warp 6. Meanwhile, the Tal Shiar assembled its own force of five  warbirds. Both fleets were manned by combat veterans, and carried enough firepower to destroy the Founders' homeworld down to its mantle within five hours. 

As part of the joint venture, the Tal Shiar carried out the assassinations of several former Obsidian Order operatives possessing sensitive information about Tain, including an unsuccessful attempt on Elim Garak. Aside from those involved in the operation, the only individuals who knew of Tain's plan were Garak and Odo, who had been captured by the Tal Shiar after investigating the deaths of the aforementioned operatives. ()

Launch 

In late 2371, the combined fleet entered Bajoran space and passed through the Bajoran wormhole. As they did so, the fleet decloaked and transmitted parallel messages to their respective governments. In Tain's message to the Central Command, he outlined his motives, intentions and advised the Cardassian military to be mobilized for potential Dominion reprisals. Both the Cardassian Central Command and the Romulan Senate denied any prior knowledge of the mission, and denounced it as a rogue operation. Despite the possibility of war, they elected to wait and see whether or not the joint operation would succeed. 

Starfleet chose not to interfere with the strike, also deliberating whether to allow Tain a chance to succeed or not. Vice Admiral Toddman ordered a civilian evacuation of Deep Space 9 and dispatched a task force of nine starships to aid in the defense of the Bajoran sector. Against orders to commit the Defiant to the defense of Bajor, Commander Benjamin Sisko took the ship into the Gamma Quadrant, in the hopes of rescuing station Security Chief Odo. ()

Jem'Hadar ambush 
 

As the fleet decloaked in orbit of the Founders' homeworld, they found the planet apparently completely undefended, with the Changelings themselves in a liquid state on the surface. Confident of victory, Tain ordered the fleet to attack, instructing; "So much for the Dominion. Open fire!"

Thirty percent of the planet's crust was destroyed in the fleet's opening volley. However, the fleet's sensors soon discovered an anomalous reading; there was no change in the number of lifeform readings on the surface. Garak soon realized the Founders had already evacuated the planet, leaving behind an automated transponder to send out false sensor readings. 

Moments later, a massive fleet of 150 Jem'Hadar fighters emerged from the Omarion Nebula, cutting off Tain's fleet entirely. The Jem'Hadar's phased polaron weaponry easily breached the shields of the Cardassian and Romulan warships, inflicting severe damage and heavy casualties in a matter of minutes. As the fleet was destroyed piecemeal, Lovok revealed himself as a Changeling to Odo and Garak, aiding their escape, unwilling to allow a fellow Changeling to die. 

Overwhelmed at being outwitted by the Dominion, Tain chose to remain on the bridge of the doomed flagship, remarking "these Founders, Elim, they're very good. Next time we should be more careful." Odo and Garak fled the crippled warbird in the runabout , immediately being set upon by Jem'Hadar fighters. On the verge of destruction itself, the runabout was spared by the timely arrival of the Defiant, destroying the runabout's pursuers and rescuing the craft's occupants.  

Hopelessly outgunned and with no hope of retreat, what remained of the Cardassian-Romulan fleet fought to its destruction. Meanwhile, the Defiant fought its way out of the battle zone, managing to escape back to Deep Space 9. ()

A few members of the fleet, including Tain himself, survived the battle and were detained at Internment Camp 371. The Romulan prisoners were rescued two years later by Garak and Lieutenant Commander Worf. Cardassian prisoners from the battle were released when the Cardassian Union formally joined the Dominion as its newest member. ()

Aftermath 
The short and long-term consequences of the battle could not be overstated. In any strategic and tactical measure, the Battle of the Omarion Nebula was a catastrophic defeat for the Romulan and Cardassian forces, and proved to have enormous ramifications for the powers of the Alpha Quadrant. 

The Tal Shiar was badly weakened, while the Obsidian Order was wiped out altogether. The power vacuum left behind by the end of the Obsidian Order allowed the Cardassian Underground to flourish. Eventually, it would launch a coup against the Central Command, restoring political control to the civilian Detapa Council. The Dominion exploited this development. Specifically, General Martok of the Klingon Defense Force was captured and replaced by a Changeling impersonator. 

The Changeling manipulated sentiment in the Klingon High Council into believing the coup on Cardassia had been engineered by the Dominion. This convinced Chancellor Gowron into launching a preemptive strike, triggering the bloody Klingon-Cardassian War. Federation refusal to support their Klingon allies, and further manipulation by Martok, led to the First Battle of Deep Space 9, and eventually the . The Federation suffered significant losses in this war and the Battle of Sector 001, while the Klingon Defense Force suffered heavy casualties in fighting both the Federation and the Cardassians.

The Cardassian Union's infrastructure was crippled by the Klingons, as well as continued conflict with the Maquis. This, coupled with political problems within the Cardassian Empire, prompted Gul Dukat to enter into secret negotiations with the Dominion. Acting as the empire's representative, Dukat negotiated Cardassia's entry into the Dominion and Dukat was named the new leader of Cardassia in 2373. The Dominion cold war would continue to escalate following this development, eventually culminating in the Dominion War. (, , )

Appendices

Background information 
When the staff writers of  came up with the idea that third season installment  would establish a military buildup which the Obsidian Order was involved in and which was so mysterious that even Cardassia's Central Command had no intel on it, the writers likewise had no idea it would lead to a battle later the same season. "We had a vague idea of a first strike," clarified René Echevarria, "but I don't think we knew more than that." It wasn't until the writers were plotting the story for subsequent third season episode  that they realized they could make the military escalation be in preparation for a preemptive attack by not only the Obsidian Order but also the Tal Shiar against the Dominion. However, even at this point, the actual battle itself wasn't planned; in fact, the Cardassians were to have been halted in their plans to combat the Dominion, due to Garak essentially blackmailing Tain. () "When Tain heard that, he thought his attack would lose the element of surprise, so he canceled the mission," Echevarria explained. (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 95) It was Michael Piller, in one of his last suggestions for the series, who encouraged the DS9 writers to "go fight the Dominion" in a second part of the narrative begun with "Improbable Cause", an episode thereafter entitled . ()

Thus, when the writers began to craft the story for "The Die is Cast", the prospect of a monumental space battle loomed. Although the creative team decided to invent a machine that could prevent a Changeling from shape-shifting, the violent engagement remained conceptually viable to the writers, as opposed to having the enemies of the Dominion rid the Founders' homeworld with the machines. Reasons why the writers still found the conflict acceptable for inclusion in the episode were that the machine was a prototype which the assailants had only one copy of and that the plans to the device were lost in the fray, also obliterating the fleet. ()

The writers felt it was important to put a lot of effort into showing the battle in "The Die is Cast". "We had set up all of this stuff about the invasion, the attack and all of those things in the first episode," remembered Ronald D. Moore, "and when we got to the production meeting, we emphasized that we were really going to do a big space battle here and wanted to see it and have it blow our socks off. We didn't want to go cheap on this one." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 95) Moore elaborated, "We knew it had better pay off. I thought, 'Wow, this could be cool, because we're telling the audience that there's an invasion fleet that's going to do this massive thing.' So we wanted as many warbirds as we could see, and a lot of Jem'Hadar ships, and we wanted the Defiant to be in the middle of them all." () Moore concluded, "The visual effects guys kind of blanched a little bit, but they went for it." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 95)

Because depicting the optical-heavy event was very expensive, Gary Hutzel was assigned to ensure the battle didn't make "The Die is Cast" go over budget. Recalling an inexpensive method he consequently employed, Hutzel said, "I shot transparencies of the Cardassian and Romulan ships on light boxes, and we 'flew' those in the background. We kept the camera in constant motion so the audience wouldn't concentrate on them." On the other hand, the visual effects workers did imagine a particularly sophisticated shot. "We wanted the scene to say that Sisko is really turning it on," reflected Hutzel, "so we designed the shot to have the Defiant fly right through a Jem'Hadar ship." ()

The Battle of the Omarion Nebula was considered a ground-breaking conflict by DS9's VFX artists, due to the extremely high quantity of ships portrayed in the incident. () Ultimately, Ron Moore was also happy with the sequence, remarking, "It's one of the best space battles we've ever done." (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages, p. 95)

External link 

cs:Bitva v mlhovině Omarion
de:Schlacht im Omarion-Nebel
fr:Bataille de la nébuleuse d'Omarion
nl:Slag om de Omarionnevel
Omarion Nebula, Battle of the