When we last saw the Mirror Universe on screen in DS9, the Terran Empire of the 23rd century had fallen, and in its place, the Klingon-Cardassian Alliance was the dominant power in the Alpha Quadrant. That began to change, in part due to the interference of visitors from the prime universe. The Terrans had started a rebellion, initially hiding out in the badlands but eventually becoming powerful enough to seize Terok Nor. Thanks in large part to the new Defiant-class starships, the Terrans were beginning to march towards victory.
In the fifteen years since the last crossover, the Terran Rebellion has made many gains, taking control of a large swath of territory, though their goal of liberating Earth . Terok Nor remains their main base of operations, and the Bajor system in the 2370s was one of the first Alliance worlds to fall. As a loyal member of the Alliance, Bajor was subjected to a brutal occupation, rivalling the brutality of the Cardassian Occupation in the prime universe. The Terrans, still a fledgling power, relied on Bajoran slave labour and resources to arm their rebellion and construct more and more Defiant-class starships. The small, fast, hard-hitting Defiant class starship is the backbone of the Terran fleet and epitomizes Terran military doctrine -- hit them hard and hit them fast.
Ruthlessly exploiting whatever opportunities they can, the Terrans have continued expanding, taking over worlds such as Gamia, Regulon, Klaestron, and even retaking the Trill homeworld in a series of bloody battles. In response to the successes of the Terrans, the Klingons and Cardassians have both vowed to wipe them out. The Klingons, seeing the success of small, agile vessels with heavy forward-firing weapons at the battle of Terok Nor, have invested in a new Bird of Prey class, which is expected to match the Defiant in speed and maneuverability.
The Cardassians have taken the opposite approach. With their cruisers facing mounting losses, they have invested in an all new class of super-dreadnought which is larger and more ambitious than anything in the quadrant and is intended to break the back of any Terran fleet. While detailed specifications are hard to come by and highly classified, the ship is bristling with defensive weaponry and numerous fighter bays, enough to fend off an attack from any direction. Its offensive weaponry is quite impressive, mounting some of the most powerful disruptor arrays in the quadrant as forward weapons. With the ability to launch a devastating planetary bombardment and follow up with the deployment of shock troops, the prospect of facing this new class strikes fear in the hearts of even the most grizzled Terran military leaders.
The lead vessel, the ACS Wrath of the Prophets, was named in honour of Bajor's contribution to the overthrow of the first Terran Empire and their participation in the Alliance. And, in an effort to restore hope to the long-suffering Bajorans, command was given to Gul Alenis, a former Colonel in the Bajoran military whose exploits during the ill-fated Battle of Gamia III earned her renown through the quadrant, at the cost of numerous crew members and her own left eye.
Meanwhile, lost in a nebula for decades, the Terrans had found an old derelict ship from the days of the Empire known as the ISS Portland. Not in a position to turn down an extra vessel, even one which is decades old, they have outfitted the Portland with the latest technology from the Defiant-class, and created a one-off vessel that can hold its own against Klingon and Cardassian light cruisers. Under the command of Captain Andrew Rouse, the ISS Portland helped turn the tide against the Cardassians at Gamia III and has continued its illustrious career as a Terran warship.