When the Terran Empire first encountered the Romulans, it could not completely subjugate them. The enigmatic Romulans demanded that the Empire left Romulan space as sovereign territory. In response, the Empire fought the Romulans to a standstill.
While Romulan spies and subterfuge provided an early advantage, Starfleet's overall technological prowess and numbers forced the hand of the Romulans. The Terrans suffered few casualties, among which were members of the influential Stiles family.
In 2160, threatened with annihilation and a desire to avoid risking total cultural contamination through an extended conflict, the Romulans offered a truce by offering to remain behind a wall of their own space that would be patrolled by Starfleet, but also stated that they would sooner destroy their own world than become subjects of the Empire.
The Empire agreed to the truce, having no desire to continue dragging out a resource-costly war and believing that the Romulans could always be subjugated later when secret technological advancements would either make the cloaking device ineffective or put it into the Empire's hands.
Following the war's end, the Romulans remained behind the Neutral Zone, only making furtive forays into Imperial space, which were easily handled by Starfleet's bases and ships along the border. (Decipher RPG module: Through a Glass, Darkly)
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.