The collapse of the Terran Empire in the mirror universe in the late 23rd century, when reforms begun by Spock left it unable to resist conquest by the Klingon-Cardassian Alliance.
In the Original Series episode Mirror, Mirror, Kirk urges the mirror Spock to change the brutal Empire before it destroys itself. The Deep Space Nine episode Crossover shows what followed.
As the mirror Kira explains to her counterpart, Spock rose to lead the Empire and introduced far-reaching reforms, including disarmament. The Klingons and Cardassians then joined forces and overran it. Terrans and their Vulcan allies became slaves, labouring in places such as the ore processors of Terok Nor, while Bajor, once a subject world, grew influential within the Alliance.
The visit of the prime Kira and Bashir in 2370 sparked the Terran Rebellion. Discovery later shows the Empire at its height and gives its eventual fall as proof that cruelty could not last.