The Dominion War was a two-year (late 2373-late 2375) interstellar conflict between the Dominion and the Federation Alliance that consumed the entire Alpha Quadrant in one of the most destructive wars in galactic history.
The seeds of war were sown as far back as 2370, when the Federation made first contact with the Dominion in the Gamma Quadrant. After the Bajoran colony on New Bajor was annihilated, the USS Odyssey was destroyed in Starfleets first encounter with the Jem'Hadar. Three other Starfleet vessels currently operating in the Gamma Quadrant were also lost: the USS Maryland, USS Proxima, and USS Sarajevo, likely destroyed by the Jem'Hadar. When a mission to speak with the Founders was undertaken, they proved to be a xenophobic and ambitious people. Their desire to control the newly-found solids of the Alpha Quadrant was obvious, but a temporary peace was brokered. The two sides entered a sort of cold war, with both participating in buildup but no aggressive actions taken.
The Dominion chose instead its tactics of subterfuge, managing to infiltrate every major Alpha Quadrant power with Founders by 2372. In 2371, the Cardassian Obsidian Order and Romulan Tal Shiar were infiltrated and encouraged to form a joint assault fleet to eliminate the Founders' homeworld in the Omarion Nebula; this proved to be a trap, and both organizations were crippled during the Battle of the Omarion Nebula, with the Obsidian Order ceasing to exist entirely.
Partly as a result of this event, the civilian Detapa Council overthrew the Cardassian Central Command as the governmental force on Cardassia Prime, and Klingon concern over the possibility of Changeling involvement in the coup led them to declare war on the Cardassian Union. The Federation Council condemned this act, and in response the Klingon Empire withdrew from the Khitomer Accords, temporarily ending the alliance and peace between the two. They attacked Deep Space Nine and pushed deep into Cardassian territory, fortifying their positions.
The Federation, meanwhile, was also in a state of high suspicion and paranoia. Founders had infiltrated Earth, and certain Starfleet elements within Federation government attempted a military coup d'etat in mid-2372. The effort was repulsed, but tensions remained high.
In early 2373, it was discovered that a changeling had taken the place of General Martok, and influenced the decision to invade Cardassia and withdraw from the Accords. After being unmasked by Benjamin Sisko and several other Starfleet officers on an undercover mission, the changeling was killed by members of the Order of the Bat'leth.
Outbreak of Hostilities
A Federation-Klingon conflict erupted in early 2373, following tensions in the Archanis sector, but it was resolved when the Dominion gained a physical foothold in the Alpha Quadrant through an alliance with Gul Dukat and the Cardassian Union, arranged in mid-2373. They immediately began sending fleets of warships and supply vessels through the Bajoran wormhole and into Cardassian territory on a weekly basis. The Klingons could not stand against such a foe, and withdrew from Cardassian space. The Khitomer Accords were put back into effect; war was inevitable, and only the Federation-Klingon alliance stood against the forces of the Dominion.
The fortification of the Cardassian Union, combined with the securing of non-aggression treaties with notable powers such as the Romulans, made Dominion intentions clear. The Federation was forced to act, but could not present the aggressive stance of something so bold as a preemptive strike. Instead, the decision was made to mine the Bajoran wormhole to prevent further Dominion reinforcements from arriving.
The maneuver was sure to provoke the Dominion into attack, and this end was achieved. The Dominion attacked Deep Space Nine and attempted to prevent the activation of the minefield. They failed, and as the minefield became active, the Federation-Klingon alliance dispatched a task force to attack the Dominion shipyards at Torros III. Their mission was successful, though the Dominion won the Battle of Deep Space Nine and held the station for the next several months. It would prove to be of little use during the war, however, as the Starfleet officers who fled the station had disabled much of its equipment, and the Bajoran contingent fomented an active (though subtle) resistance.
The early part of 2374 found the Dominion trying its hardest to disable the minefield that stood in front of the wormhole. By the second quarter of that year, it was known that they were close to achieving this, and Captain Benjamin Sisko told Starfleet that the station's recapture must be seen as the top priority of the war. The station was retaken (with the aid of the Prophets) during Operation Return. It was the first major alliance victory of the war, and spelled the end of Dukat's leadership of Cardassia. His underling, Damar, replaced him.
However, even with this victory, the Federation and Klingons were struggling to hold their own. The Dominion took Betazed in 2374, and it was clear that more help would be needed if the alliance had any hope of victory. The Romulan Star Empire had to this point declared itself neutral in the conflict, though they were allowing Dominion ships passage through their space. Captain Sisko undertook an act of coercion with the help of Garak, implicating the Dominion in an assassination plot against the Romulan proconsul. His ploy was successful, and the Romulans joined the alliance.
It was with this added firepower that the alliance was able to win the First Battle of Chin'toka in 2374, taking the strategically important system in their first major incursion into Dominion territory.
One very important event was changing the shape of the war away from the front lines: a debilitating virus was attacking the Founders themselves. It would later be claimed that Section 31 may have had a hand in the infection. The Founders' abilities as leaders were being compromised just as their military was taking severe punishment from the emboldened alliance.
Counterattack and Victory
Indeed, with Romulan reinforcement, the alliance had forced something of a stalemate. Early and middle 2375 was spent with neither side offering a firm push forward into the other. To add to the Dominions troubles, Damar was openly criticizing the war effort and deriding the Founders' leadership. It seemed to him that Cardassian troops were withstanding the worst of the assault, and statistics proved him correct. Dominion leaders decided that a change was necessary, and in late 2375 they sought the help of the Breen, a mysterious but powerful race. In the ensuing months, the Breen took more and more Cardassian responsibility, leadership — even territory. The tide had turned in favor of the Dominion, but Cardassians grew more restless by the day.
With assistance from Breen energy-draining weapons, the Dominion won the Second Battle of Chin'toka and regained control of the system. Starfleet and Romulan vessels were decimated by the new technology, forcing the Klingons — whose ships were immune to the weapon — to hold the front lines until a countermeasure could be found. The Breen even staged an attack on Earth (the Raid on San Francisco), dealing a heavy blow to Federation morale.
The Dominions obvious favoring of the Breen finally frustrated Damar to the point that he formed an active resistance on Cardassia. He first attempted a military coup d'etat, but Gul Revok betrayed him and the rebellion was quickly shut down. Damar, with help from Colonel Kira Nerys and Garak, transitioned to an underground civilian resistance. It was a success, and public anger against the Dominion was rising by the day.
The alliance also successfully countered the Breen energy weapon, and with this, the tactical advantage returned to their side. The Dominion withdrew to Cardassian space with designs on rebuilding from behind fortified borders. The determination was made that the best hopes of success for an invasion were now, before the Dominion was allowed to strengthen itself any further. A three-pronged attack on Cardassia Prime was to be led by Captain Sisko, Admiral William Ross and now-Chancellor Martok.
The Cardassian resistance was alive and well, and knew of the impending invasion. They planned massive work stoppages and sabotage to prevent the Dominion from having ground support during the battle. The sabotage of power to the Dominion headquarters prompted a swift and brutal reprisal — the massacre of the population of Lakarian City.
The Battle of Cardassia was one of the largest space battles in galactic history, and was won by the alliance with unexpected help — Cardassian commanders received the news of Lakarian City's fate and turned on the Dominion. The Dominion responded typically — the head Founder ordered the Jem'Hadar to offer no quarter and fight to the last man, as well as commanding the annihilation of the entire planetary population of Cardassia Prime. Finally, Odo was able to break into Dominion headquarters with Kira and Garak, and he convinced the head Founder to give up the war by offering to return to the Great Link and cure the virus that had been crippling his people. The war was officially ended with the signing of the Treaty of Bajor, and all Dominion forces returned to the Gamma Quadrant.
Foremost in the aftermath, however, was the state of Cardassia Prime. Eight hundred million civilians were dead, and the entire Cardassian Union was in danger of total collapse. The Alpha Quadrant had lost a major power, and the galactopolitical ramifications of the war would continue to be seen for many years to come.
This page is based in large part on the Dominion War page at Memory Alpha. All credit goes to the people who worked on that excellent article.