Klingon-Cardassian Alliance

In the mirror universe, the Alliance was a union of the Klingon and Cardassian peoples forged by Emperor Tiberius to conquer the Terran Empire following his ousting by Spock (mirror). No one else had thought that pairing possible, for while it was in the Cardassian nature to seek alliances, it was not a common Klingon trait, except under exceptional circumstances. (Hardcover: Spectre)

In 2370s, the Terran Rebellion was formed by Benjamin Sisko (mirror) to fight against the Alliance. (DS9: "Through the Looking Glass", "Shattered Mirror", "The Emperor's New Cloak")