The Dominion was an evil interstellar organization that exists in the Star Trek universe.
The history of the Dominion begins with the history of the Founders as recorded in the memory of the Great Link. Several millennia ago, Changelings were a monoformed race who were given morphogenic abilities by a giant Changeling called the Progenitor. They roamed the stars searching out other races so they could add to their knowledge of the galaxy. They went in peace but were too often met with suspicion, hatred and violence. Solids, fearing the Changelings' metamorphic abilities, beat, hunted and killed them. According to Dominion legend, on one planet, a group of Vorta – a species of small, timid, ape-like forest dwellers living in hollowed-out trees – hid a Changeling from an angry mob, and in return the Changeling promised that one day they would be transformed into powerful beings and placed at the head of a vast interstellar empire. Following their retreat to the Founders' homeworld inside the Omarion Nebula, the Changelings sought to ensure that they would never again be at the mercy of the solids by imposing "order upon a chaotic universe" around them.
Domination of the Gamma Quadrant
The founding of the Dominion, itself, occurred between 8000 BC and 4th century AD. It was at this time that the Founders first employed Vorta as their administrators, diplomats, scientists, and commanders. To enforce their will, the Founders engineered the Jem'Hadar, a species specifically designed for warfare. The Jem'Hadar became the Dominion's first line of defense. The Vorta and the Jem'Hadar under their command rapidly began to conquer space, in the name of the Founders.
Over time, the Founders sent out 100 infant Changelings in order to continue gathering information and explore. They also hoped that the activities of these 100 infant Changelings would draw the attention of the Progenitor to return and replenish the species. It was genetically coded into these infants for them to return later and share what they have learned. Odo and Laas were amongst them. Odo was the first to return - his return was some 300 years before the Dominion expected.
The Dominion's ability to mass-produce soldiers, weapons, and ships gave them a substantial advantage, particularly in wars of attrition. Due to their very long lifespans, Changelings took the long view, and were very patient in achieving their goals.
In the mid-22nd century, the Dominion encountered a space-faring race in Teplan system. The Teplans believed nothing was beyond their ability, including resisting the Dominion. In response, the Jem'Hadar came and left their homeworld crushed, as an example to others. More than anything, the Dominion wanted the Teplans to bear the mark of their defiance, so they unleashed a biological weapon, establishing the disease known as the blight. The Dominion also made a similar example of the inhabitants of Boranis III. Around 2340, the Dominion conquered a race, native to Yadera Prime, known as the Yaderans. By 2370, the Jem'Hadar invaded and conquered a race known as the T-Rogorans.
Contact with the Alpha Quadrant
The first official contact between the Dominion and the United Federation of Planets came at the end of 2370, when Jem'Hadar forces destroyed a number of ships and colonies from the Alpha Quadrant side of the Bajoran wormhole and took Benjamin Sisko and Quark hostage. The Jem'Hadar claimed that the offensive was in response to incursions into their territory. However, the offensive was actually a cover to plant Eris, a Vorta spy, into the Federation. The attempt was unsuccessful, though Eris escaped back to the Dominion. The Dominion-Federation Cold War was officially begun. In the next year, the Dominion ascertained the resistance of the major powers in the Alpha and Beta Quadrants to a Dominion invasion through an elaborate simulation involving the captured members of the USS Defiant. Based on these simulations (which showed that Starfleet would destroy the wormhole if necessary), the Dominion adopted a more subtle approach. Changelings infiltrated various powers, including the Federation, the Klingon Empire, and the Romulan Star Empire, posing as high-ranking officials. These agents maneuvered these powers into several destructive conflicts to erode their strength. Between 2371 and 2373, only a few of these infiltrators were ever exposed: one posed as Ambassador Krajensky, one posed as General Martok, and another one posed as Doctor Julian Bashir. None of the four changelings on Earth could be apprehended despite sweeping security precautions.
The Dominion War
In 2373, the Dominion vastly solidified its foothold in the Alpha Quadrant by incorporating the Cardassian Union, in a pact negotiated by Gul Dukat. In the following months the Dominion sent massive numbers of ships and troops through the wormhole into Cardassian space, after the changeling posing as Bashir sabotaged the Federation's attempt to collapse the wormhole. In addition, the Dominion massacred the Maquis and signed nonaggression pacts with the Miradorn, the Tholian Assembly, Bajor, and the Romulan Star Empire.
Although Dominion officials claimed their convoys were meant to help rebuild Cardassian infrastructure, Starfleet Command was unconvinced and, with the Dominion rapidly making political in-roads, ordered Captain Sisko to block the Alpha Quadrant entrance to the wormhole. Open war finally broke out when Starfleet mined the entrance of the wormhole to prevent any further Dominion reinforcements from coming through. The first part of the war, in early 2374, was a disaster for the allied Federation and Klingon forces, which were forced into retreat on nearly all fronts. The war turned in the allies' favor when a massive fleet of 2,800 Jem'Hadar warships, which would have ensured a Dominion victory, disappeared inside the wormhole. This unexpected event allowed the recapture of the strategic starbase Deep Space 9 by the allies. The Dominion's situation was further worsened when the Romulans abrogated their treaty and joined the war against them later that year.
Meanwhile, the Founders and their Great Link found that they had been infected with a morphogenic virus, which threatened to destroy the very foundation of the Dominion. Despite the resources at their disposal, the Founders could not discover a cure. The Female Changeling, trapped in the Alpha Quadrant, forged an alliance with the Breen Confederacy in late 2375 to win the war before her death. However, the terms of their treaty of alliance were extremely unfavorable to the Cardassians, which finally led to a rebellion. The subsequent betrayal of the Dominion fleet by the Cardassians at the Battle of Cardassia, which came as a result of Dominion reprisals against Lakarian City, would seal defeat for the Dominion in the war. Surrounded by allied forces on Cardassia Prime, the dying Female Changeling defiantly ordered her forces to fight to the death, believing that to show weakness would lead to the solids invading the Dominion in the Gamma Quadrant. She eventually agreed to surrender after Odo linked with her and cured her of the virus. Odo subsequently returned to the Great Link to cure the rest of his people, while the Female Changeling remained in the Alpha Quadrant to assume responsibility for the war.
After Odo returned to the Great Link he was able to cure the other Founders from their illness. The other Founders decided to remain within their current borders for the time being, and informed the Alpha Quadrant they were welcome to explore the Gamma Quadrant. However Odo was not able to convince the Great Link to drop their distrust of the Solids, or in granting more freedom to the Dominion.
Laas soon joined them in the Great Link, bringing back a couple more members of the 100, including one who had died. Laas would often be responsible for relaying the decisions of the Link to Odo as Odo spent a great deal of time separate and in his humanoid form.
The Great Link dissolved in 2376 after the death of the Progenitor. Unable to cope with their failure to bring the Progenitor back to them, the majority of the Founders departed to seek solitude. This left Odo, Laas, and a few other Changelings in charge of the Dominion.
Over the next several years Odo worked to transform the Dominion into a more democratic and peaceful society while Laas worked to find the members of the Great Link and other members of the 100. Due to the structure of the Dominion where the majority of the population never encountered a Founder little changed for the member races. Odo closed the borders to keep the political situation stable in the meantime.