First Contact and the Growth of Hostilities
The Dominion were an unknown race to the Alpha Quadrant powers until the discovery of the Bajoran wormhole in 2369, which facilitated exploration of the Gamma Quadrant. In 2370 Jem'Hadar troops annihilated a Bajoran colony in the Gamma Quadrant and captured Commander Sisko of the starbase Deep Space Nine. The lead Federation rescue ship, USS Odyssey, was destroyed by a kamikaze attack, as the Dominion dramatically demanded that Federation explorers stay out of their end of the galaxy. (DS9, "The Jem'Hadar")
A Federation mission the next year to find and make peace with the Founders ended disastrously. Founders began infiltrating the Alpha Quadrant, even wreaking havoc on Earth itself. The quadrant was plunged into conflict when the Klingon Empire accused the Cardassian Union of being under the control of the Founders. The Federation and Cardassians fought months of armed combat against the Klingons. It was later revealed that it was the Klingons themselves who unknowingly had a Changeling in their midst.
The Dominion War (2373-2375)
The Dominion gained a foothold in the Alpha Quadrant when Gul Dukat announced that the Cardassian Union was joining the Dominion in 2373. Open hostilites began a few months later when the Dominion attacked and occupied Deep Space Nine, at the mouth of the Bajoran wormhole, while Federation and Klingon allied forces invaded and destroyed a Dominion shipyard. The Allies suffered brutal losses for over four months, until Captain Sisko organized a task force to retake Deep Space Nine. A Dominion fleet intercepted the task force just outside of Bajoran space, but it was flanked in a spectacular battle, one of the largest in the Federation's history. After the major upset of the Allied victory at Bajor, along with the destruction of a force of 2800 Dominion ships due to the intervention of the wormhole aliens known as the Prophets of Bajor, the war ground on for over a year with neither side gaining a clear advantage. The Romulan Star Empire entered the war on the side of the Allies in 2374.
A Cardassian rebellion against Dominion rule formed shortly thereafter under the leadership of Legate Damar, Dukat's successor. The Dominion was finally defeated when, during a final Allied onslaught on Dominion forces in orbit around Cardassia, the Cardassian ships switched sides and aided the Allied fleet. The Dominion, before surrendering, launched a genocidal assault on the Cardassian people in retaliation, killing over 800 million Cardassians. (DS9, "What You Leave Behind")
The Founders themselves were nearly wiped out by a plague, which was revealed to be a biological weapon engineered by agents of the clandestine Federation agency Section 31, who deliberately infected Odo with it, in the expectation that he would pass it through the great link. After the end of hostilies, the Founders were cured.