Klingons, Mirror Universe

Star Trek: Deep Space Nine
Episode: DS9 443 - Crossover

Planet: Mirror Qo'noS. Klingons are still ferocious and stout warriors in the Mirror Universe, where they have entered in to an Alliance with the Cardassians there after defeating the Terran Empire — mostly fulfilling security and defense positions. Uniforms, demeanor, equipment and ships all appear to be exactly the same as in the known universe.