In at least two iterations of the mirror universe, the Earth-Romulan War was, as with the Earth-Romulan War which was fought in the primary universe, an interstellar war fought between the Terrans and the Romulans in the 22nd century.
In a permutation of the mirror universe in which Admiral James T. Kirk took the USS Excelsior to the mirror universe and pretended to be his counterpart, the Romulans' conflict with the Terrans was long-standing, as the mirror Romulans had also engaged in a 22nd century Earth-Romulan War against Earth, in which the Romulans had briefly established a beachhead on Earth and enslaved Terrans. The Terrans were able to overthrow the Romulan rule and fight them out of their space, establishing the strong interstellar Terran Empire seen in the 23rd century. (TOS - The Mirror Universe Saga comics: "Masquerade!", "The Beginning of the End...")
In another permutation of the mirror universe in which the Terran Empire survived well into the 24th century, the Romulans had been cleansed from their homeworlds by 2367. The Earth-Romulan War did not end with just the Romulans' humiliating defeat at the Battle of Cheron, but rather Imperial vessels fought all the way to Romulus and, after destroying Remus, offered the Romulans opportunity to become a subject race of the Terran Empire. On the eve of the deadline for the Romulans' reply, the entire population of the planet committed mass suicide. The Terrans gained numerous technological advancements from the sacking of Romulus, including the molecular disassociator. (TNG novel: Dark Mirror)
- The date of the Romulan defeat is not noted, but is implied to be at the end of the 22nd century conflict with Earth. The implication of their defeat obviously contradicts every source thereafter where the Romulans are shown alive, unless the government somehow continued and regained strength following the loss of the homeworlds. The source material in Dark Mirror contradicts the canon version of the mirror universe in many other ways, however.