Star Trek: Deep Space Nine revealed the prime timeline Captain James T. Kirk (William Shatner) broke the Mirror Universe during his one and only visit. At the end of Star Trek: The Original Series season 2, episode 10, “Mirror, Mirror,” Kirk suggests to Mirror Universe Spock (Leonard Nimoy) that the future of the Terran Empire could be more peaceful, like the Federation in Star Trek‘s prime timeline. Star Trek didn’t return to the Mirror Universe for another 27 years, in DS9 season 2, episode 23, “Crossover”, which aired in 1994, and revealed a number of details about how Kirk impacted the Terran Empire.

Having rejected multiple Mirror Universe storylines for Star Trek: The Next Generation, producer Michael Piller eventually gave into his curiosity in Star Trek: Deep Space Nine. Various Star Trek: DS9 characters crossed over into the Mirror Universe between seasons 2 and 7, beginning with “Crossover”. DS9‘s first Mirror Universe episode established what happened after Mirror Spock attempted to implement the changes suggested by Prime Kirk. Spock’s attempts to reform the Terran Empire were a disaster, and created an even harsher dystopia than before.

DS9’s Mirror Universe Episode Revealed Captain Kirk Broke The Alternate Timeline

In Star Trek: Deep Space Nine season 2, episode 23, “Crossover”, Dr. Julian Bashir (Alexander Siddig) and Major Kira Nerys (Nana Visitor) are transported to the Mirror Universe following an accident in the Bajoran wormhole. In the 24th century Mirror Universe, there is no Deep Space Nine, but there’s a Terok Nor, which is overseen by the Intendant, the Mirror version of Kira. As with its prime universe counterpart, the Cardassian space station Terok Nor is an ore processing plant populated with Terran slaves.

It’s revealed that Kirk had a “profound influence” on Mirror Spock, who rose to commander in chief of the Terran Empire with promises of more peaceful reform. This substantially weakened the Terran Empire so that it was unable to defend itself against the Klingon-Cardassian Alliance. The Terran Empire fell, and the Klingon-Cardassian Alliance became the dominant force, meaning that Kirk ultimately seeded the downfall of the Mirror Universe. Star Trek: Discovery season 5, episode 5, “Mirrors” revealed that Mirror Spock was executed for introducing his Kirk-influenced reforms, presumably when it was realized the Terrans could not defend themselves against the Klingon-Cardassian Alliance.

Why DS9 Brought Back Star Trek’s Mirror Universe

Star Trek: TNG didn’t do the Mirror Universe, largely because its pulpy idea wasn’t in-keeping with TNG‘s more serious tone. In the book Captains’ Logs Supplemental – The Unauthorized Guide to the New Trek Voyages by Edward Gross and Mark A. Altman, Michael Piller revealed his thinking behind bringing back the Mirror Universe in Star Trek: Deep Space Nine, and it was all because of Captain Kirk:

I couldn’t get away from the fact that it would be interesting to know what happened after “Mirror, Mirror” finished. I couldn’t escape the idea that Kirk’s influence in the world that he left might have been profound and changed history. What would be more of a gross violation of the Prime Directive?

Robert Hewitt Wolfe, who contributed the idea of the fall of the Terran Empire told the Star Trek: Deep Space Nine Companion that he wanted to explore why the Terrans were so brutal in Star Trek: The Original Series. Hewitt Wolfe’s belief was that the Terrans had made themselves harsh and brutal to protect against the “barbarians at the gate“; the Klingons and Cardassians. Star Trek: Enterprise would later dismiss this idea, by revealing that the Terrans were cruel and brutal long before First Contact with the Vulcans. However, without Star Trek: Deep Space Nine revitalizing the concept, it’s possible that Enterprise‘s Mirror Universe two-parter may never have happened.