When the Cardassian occupation of Bajor ended in 2369, the mining space-station Terok Nor was left abandoned, its systems ripped out. By invitation of the provisional Bajoran government, Starfleet stepped in to oversee the rebuilding and day-to-day operations of the newly christened Deep Space Nine. Starfleet's position was a tentative one, many Bajorans suspicious and unwelcoming as a result of Cardassian oppression and brutality. However the alliance held and soon DS9 was a center of travel and commerce thanks to a newly found stable wormhole, leading to the largely unexplored Gamma Quadrant.
Then after two years, the Dominion - a hierarchy of three separate species, the xenophobic Changelings, the diplomatic but shifty Vorta and genetically-bred soldiers, the Jem'Hadar - made their presence known. Seeing these new travelers coming through the wormhole as a threat to their power base, they decided to bring their version of order to the Alpha Quadrant... by conquering it.
So began the war. At first fought covertly, then in plain sight, the war would soon drag each of their major powers in as their governments struggled against paranoia and shifting loyalties, thanks to Changeling infiltrators. This war wasn't for territory or power, it was for freedom.