The Dominion War was the bloodiest conflict in Star Trek history, and its lengthy timeline was anything but straightforward. Officially started in 2373, the storm that would eventually become the Dominion War had been brewing since the early days of Star Trek: Deep Space Nine. Though the Federation and Starfleet actively avoided direct conflict with the Dominion for years, war itself became an eventuality thanks to the Founders' unquenchable thirst for power. While the Star Trek franchise had been notable for its depiction of a peaceful future, wars weren't entirely unprecedented.
Essentially a fight between the combined forces of the Alpha and Gamma Quadrants, the war was also a battle of ideas that showed just how alien the distant Gamma Quadrant was from the more familiar Alpha. Though there had been plenty of wars in the Star Trek franchise, the Dominion War elevated far beyond the usual cold standoffs of the past and was a shooting conflict for several years. Along the way, the Federation's greatest alliances were tested, and they were forced to forge new bonds that were hitherto thought impossible. Though the Alpha Quadrant was victorious, the fallout of the war shaped the Star Trek franchise going forward.
Before The War (2369 - 2373)
Almost immediately after the discovery of the stable wormhole near Deep Space Nine in 2369, rumblings of the secretive Dominion began to spread thanks to those who had ventured into the Gamma Quadrant. Official first contact was made with the Dominion in 2370 when a Jem'Hadar ship destroyed Starfleet's USS Odyssey. That overt act of aggression created a state of cold war between the parties, but it wouldn't stay cold for long. It was soon discovered that the Founders were in fact Changelings like Odo, and they used their transformative powers to destabilize the government of Cardassia, which eventually led to a power vacuum and coup.
Changeling infiltration into the Klingon government lead to the Klingon/Cardassian War in 2372, and the Federation condemned their Klingon allies. This disagreement dissolved the Khitomer Accords and led to the Klingon/Federation War the same year. Also in 2372, the Founders nearly succeeded in destabilizing Earth when they bombed the Antwerp Convention that almost led to a military coup on the planet. Under the leadership of Gul Dukat, Cardassia broke away from the Alpha Quadrant and allied themselves with the Dominion, and the Federation's plan to permanently seal the wormhole was thwarted by the Changelings. Peace between the Klingons and Federation was reestablished in 2373, on the eve of full-scale war.
War Is Declared (2373 - 2374)
After Worf smoothed over problems with the Klingons, the decision was made to place a self-replicating minefield at the mouth of the wormhole, an act that was an active declaration of war. Almost as soon as war began, the Alpha Quadrant Allied forces suffered heavy losses, and Deep Space Nine was seized after the Allies retreated from the Bajoran system. The battle in the Tyra system in 2374 saw the Allies suffer a crippling defeat, and morale began to sag. Even as the Allies began making key attacks on the Dominion's supply chain and communications network, the war quickly became a losing effort with thousands perishing.
The Counter Attack (2374)
With options dwindling and defeat seemingly on the horizon, Captain Benjamin Sisko implored Starfleet to allow him to organize a counter-attack to retake Deep Space Nine. Many of the ships in Starfleet and other Allied ships joined the offensive, but the Klingon Empire was reticent to dedicate forces and leave their territory vulnerable. Without Klingon assistance, Sisko led the offensive to prevent the destruction of the wormhole minefield and was outgunned before the sudden arrival of the Klingons. The combined forces were able to defeat the Dominion fleet, but the minefield was successfully disabled. Sisko then convinced the Prophets to eliminate the Dominion forces in the wormhole to protect Bajor.
Turning Point For The Allies (2374 - 2375)
Following the defeat at Deep Space Nine, and without the ability to return to the Gamma Quadrant, the Dominion retreated to Cardassian space and began negotiating with the Allies. Peace talks were stalled, and active conflict soon started again in earnest with the Dominion ramping up their production of ships and new Jem'Hadar soldiers. With characters like Nog living through the darkest episodes of the war, the Dominion attacks led to larger and larger casualty lists as Allied morale again began to decline. The Dominion then seized control of the Federation planet Betazed, which was a massive strategic blow to the Alpha Quadrant.
The Romulan Star Empire had remained neutral during the war up to that point, but Benjamin Sisko turned the Dominion's own tactics against them when planting false information within Romulan circles that implicated the Dominion in an assassination. That subterfuge turned the Romulans to the Allied side, and they immediately launched a counter-offensive against the Dominion near their space. It was soon discovered that a Morphogenic virus had been implanted within the Changeling's great link by characters in the Federation's Section 31, and though a cure was developed by Dr. Bashir, the Allies opted not to divulge it.
The Breen Join The Dominion (2375)
Following the crack-up and decline of Gul Dukat in 2374, Gul Damar became the leader of the Cardassians after rising to power. By 2375, relations between the Cardassians and the Dominion had become strained, as the latter viewed Cardassian forces as expendable, and even blamed them for defeats. In 2375, the Dominion accepted an alliance with the Breen which further lowered the standing of Cardassians and eventually led to a military rebellion within Cardassian forces. Bolstered by weapons and training from the Allies, the Cardassian rebellion attempted to weaken the Dominion but was betrayed by Gul Revok, who was a double agent.
Meanwhile, one of Star Trek's most influential Klingons, Chancellor Gowron, began to resent General Martok for what he interpreted as designs against his rule, and he sent the general on a series of dangerous missions. Worf was dispatched to quell the internal struggle, and he was forced to slay the reigning chancellor in combat to install Martok as the new leader of the Klingon Empire. During the crushing defeat of the Allies in the Chin'Toka system, the Klingon ship IKS Ki'Tang discovered a way to resist the Breen's energy weapons by adjusting their tritium intermix, and word was dispatched to the entire Allied fleet to make the adjustment.
The War Concludes (2375)
Following the discovery of a novel solution to the Breen's energy weapons, the Allied forces were able to serve decisive defeats to the Dominion and finally drove them back to their final territory within Cardassian space. In an ironic twist on the Cardassian occupation of Bajor, the defeated Cardassian rebellion inspired the civilian populace of Cardassia to rise up against the Dominion, and the planet itself was destabilized from within. Opting not to give the Dominion a respite, the Allied forces launched a massive attack on Cardassian space, and Odo was able to convince the Founders to surrender in exchange for the cure to the Morphogenic Virus.
A few days after the surrender, in late 2375, representatives of the Federation and Dominion signed the Treaty of Bajor that returned all boundaries in the Alpha Quadrant to their positions before the Dominion began expanding that area of space. The Dominion was ordered to remain in the Gamma Quadrant, and Odo kept up his end of the bargain by delivering the cure. Though Star Trek: Nemesis' Romulan scenes were disliked, the film told the story of the Star Empire in the immediate years after the conflict and made it clear that the Star Trek universe would take a long time to recover from the Dominion War.