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.
Background
The year 2371 held high tensions for the powers of the Alpha Quadrant. The Dominion threat, though they were on the far side of the Bajoran wormhole, began to grow.
The Jem'Hadar attacks in 2370, and an engagement between the USS Defiant and several Jem'Hadar fighters, had 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 encounter with the Jem'Hadar, war seemed to be inevitable.
Alpha Quadrant powers began to prepare for invasion, and assigned the Defiant to Deep Space 9 to supplement 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, the former head of the Obsidian Order was living out his retirement. Frustrated by what he perceived to be the Cardassian Central Command's complacency in the face of the Dominion threat, the man began a risky venture to destroy the Dominion in a single decisive move: wiping out the Founders.
Enlisting the aid of the Romulan Tal Shiar, he proposed a joint military agreement into the Gamma Quadrant, which was to be executed independently of either's government. However, his Tal Shiar counterpart had been replaced by a Changeling. Via the Romulans, the Changeling now had access to all the intelligence gathered by Starfleet on the Dominion.
The Obsidian Order began assembling a fleet, even though Cardassian law specifically forbade the Order from operating military spacecraft of any kind. The fleet was discovered Central Command, during an incident involving the Defiant and the Maquis. Equipped with cloaking devices provided by the Tal Shiar, the fleet had modified these devices to help them avoid detection by Dominion sensors, though this was ineffective above warp 6. Meanwhile, the Tal Shiar assembled its own force. Both fleets were manned by combat veterans, and carried enough firepower to destroy the Founders' homeworld.
As part of this joint venture, the Tal Shiar carried out assassinations of several former Obsidian Order operatives.
Attack
In late 2371, the Cardassian-Romulan mini-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. Both the Cardassian Central Command and the Romulan Senate immediately denied any prior knowledge of the mission, and denounced it as a rogue operation.
Starfleet chose not to interfere with the operation, waiting with the other governments to see if the plan would succeed. A civilian evacuation of Deep Space 9 was ordered, and nine starships were sent to aid in the defense of the Bajoran sector.
Jem'Hadar Ambush
As the fleet arrived in orbit of the Founders' homeworld, they found the planet undefended, with the Changelings themselves in a liquid state on the surface. Confident of victory, the fleet attacked.
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. The Founders had already evacuated the planet, leaving behind an automated transponder to send out false sensor readings.
Moments later, a massive fleet of Jem'Hadar fighters emerged from the Omarion Nebula, cutting off the joint fleet entirely. The Jem'Hadar's weaponry easily breached the shields of the Cardassian and Romulan warships, inflicting severe damage and heavy casualties in a matter of minutes. Hopelessly outgunned and with no hope of retreat, what remained of the Cardassian-Romulan fleet fought to its destruction.
A few members of the fleet, survived the battle and were detained at an internment camp. The Romulan prisoners were rescued two years later, and Cardassian prisoners were released when the Cardassian Union formally joined the Dominion as its newest member.
Aftermath
The Battle of the Omarion Nebula was considered 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 space left behind by the end of the Obsidian Order allowed the Cardassian Underground to flourish, leading to an eventual coup against Central Command. Political control was then restored to the civilian government known as the Detapa Council.
A Changeling took over key political figures in the Klingon government, manipulating the Klingon High Council into believing the coup on Cardassia had been engineered by the Dominion. This triggered the Klingon-Cardassian War. Federation refusal to support their Klingon allies led to the First Battle of Deep Space 9.
The Cardassian Union's infrastructure was crippled by the Klingons, as well as their continued conflicts with the Maquis. These events, among others, prompted one Cardassian to enter into secret negotiations with the Dominion. Acting as the empire's representative, Cardassia entered into the Dominion in 2373.
