(coexisting on another dimensional plane)
In the mirror universe, the Terran Rebellion was an uprising by the Terran slaves of the Klingon-Cardassian Alliance in the early 2370s. Against overwhelming odds the rebellion, which came to be called the Terran Resistance Forces, scored several important victories against the Alliance and restored hope to the defeated slaves.
In 2370, incited by stories from Julian Bashir and Kira Nerys of a free Humanity in the "prime" universe, Terran captain Benjamin Sisko rejected the authority of Intendant Kira Nerys and escaped from Terok Nor aboard his small raider. (DS9: "Crossover")
The charismatic Sisko managed to raise a full-scale rebellion, gathering together a band of troops and ships based in the Badlands. Although Sisko was killed trying to reach Terok Nor to convince his former wife, Jennifer Sisko, to join the Terran cause, the rebellion continued on, led by Miles "Smiley" O'Brien, with a little help from the "prime" universe version of Sisko. (DS9: "Through the Looking Glass")
In 2372, the rebels scored their first major victory: the capture of Terok Nor from the Alliance. In order to maintain their power and defend their new conquest, the rebels built a new warship, a replica of the Starfleet warship USS Defiant, which they also named Defiant. This powerful little ship was successful in defending the newly-captured station at the Battle of Terok Nor. (DS9: "Shattered Mirror")
The rebels, now calling themselves the Terran Resistance Forces, won a great victory in 2375, when the ISS Defiant, thanks to sabotage by the "prime" version of Rom, managed to capture Regent Worf, the leader of the Alliance. When last seen, the rebels' march towards total victory appeared to be well under way. (DS9: "The Emperor's New Cloak")