Dominion War

The Dominion War was a major interstellar conflict, fought from 2373 to 2375, though related conflicts began earlier. The war involved all major powers of the Alpha and Gamma Quadrants, formed into two opposing military alliances; the Federation Alliance and the Dominion-Breen Alliance. Marked by massive civilian casualties, including the Jem'Hadar bombardment of Cardassia (in which approximately 800 million Cardassians were killed), and the mobilisation of military fleets on a scale never seen prior, it was one of the bloodiest and most destructive wars in galactic history.

After a series of calculated steps to destabilise the political landscape of the Alpha Quadrant, the Dominion launched an attack on Deep Space 9 after the Federation provocatively mined the Bajoran wormhole. After a sluggish start, the Dominion rapidly conquered vast swathes of Federation and Klingon territory. In mid-2374, Allied forces recaptured Deep Space 9, though the Dominion soon recovered, conquering Betazed.

After the death of Senator Vreenak, the Romulan Star Empire declared war on the Dominion in late 2374. Allied forces launched a joint invasion of Cardassian space late in the year. Requiring additional manpower, the Dominion signed a treaty with the Breen Confederacy, and, with their aid, launched a raid on Earth, and recaptured the Chin'toka system.

Political instabilities within the precarious Dominion alliance triggered a major rebellion by Cardassian military forces, rendering the Dominion position untenable. As the Dominion-Breen Alliance began to unravel, the Allies launched a massive invasion of Cardassian space in late 2375, sealing the final defeat of the Dominion.

The Treaty of Bajor formally ended the fighting in late 2375. Source: Memory Alpha