The Dominion War was a two-year interstellar conflict between the Dominion and the Federation Alliance, from late 2373 to late 2375, that consumed the entire Alpha Quadrant in one of the most destructive and bloody wars in galactic history. By the end of the war, the Alpha Quadrant had lost a notable power – the Cardassian Union – the Klingon Defense Force was set back for decades, and there remained a catastrophic death toll for all powers involved.
Following the Federation's first official contact with the Dominion in 2370, a cold war developed between several of the Alpha Quadrant's major powers and the Dominion that lasted for three years. By placing Changeling infiltrators in important positions within the governments of the Alpha and Beta Quadrant's major powers, the Dominion was able to wipe out the Cardassian Obsidian Order, drastically reduce the capabilities of the Romulan Tal Shiar, nearly cause a war between the Federation and the Tzenkethi, and start a war between the Klingon Empire and the Cardassian Union, an act which concurrently caused a conflict between the Federation and the Klingons.
The conflict between the Klingons and the Cardassians is what paved the way for the Dominion to gain a foothold in the Alpha Quadrant. In mid-2373, Gul Dukat, frustrated with the losses the Cardassians had taken from the Klingons, negotiated the entry of the Cardassian Union into the Dominion. The Dominion immediately began fortifying the Cardassian Union with weekly convoys of warships and troops from the Gamma Quadrant. With his new Dominion allies, Dukat was able to make good on a promise to drive the Klingons from Cardassian space and wipe out the Maquis resistance in the Demilitarized Zone. At this point the Federation and Klingons renewed their alliance and agreed to stand side-by-side against the new Dominion-Cardassian threat.