|Emblem of The Terran Empire|
The Terran Empire was a repressive Terran-dominated government and mirror universe counterpart of the United Federation of Planets. (Star Trek: The Original Series, Star Trek: Deep Space Nine, Star Trek: Enterprise)
While in the mirror universe, after being accidentally transported there, James T. Kirk was under orders to annihilate the Halkans, who refused to allow the Empire to mine their dilithium. The Halkans feared it would be used for the Empire’s war capabilities, and they warned that one day the Empire’s brutal policies would lead to its collapse in a devastating civil war. Soon, the mirror universe Spock came under orders to assassinate Kirk if he did not comply within a few hours. Spock warned Kirk of those orders, and ,realizing Spock was a man of integrity in both universes, Kirk saw in Spock a chance to reform the Empire and try to avert its predicted collapse. Before returning to his own universe Kirk urged Spock to try to steer the Empire and the mirror universe toward a more civilized state. Spock said he would “consider it.” (TOS: "Mirror, Mirror")
- Unknown to the Federation, another contact occurred a century earlier from the Empire's perspective, when the Tholians detonated a tricobalt device, creating an interphase between the two universes. (ENT: "In a Mirror, Darkly")
It was learned a century later that Spock did more than consider it, but whether he brought civilization to the Empire that universe remained as violent and barbaric as ever. It was related to Kira Nerys by her mirror counterpart that Spock had apparently disarmed the Empire and left it vulnerable to conquest by an alliance of Klingons and Cardassians, and that humans and Vulcans had become slaves. (DS9: "Crossover")
Dark Mirror timelineEdit
A few years after Spock began effecting some changes, Spock was set up on treason charges and executed before he could reform the Empire, which had wiped out the Romulan race and conquered the Klingons. (TNG novel: Dark Mirror)
- This empire was never conquered by the Klingon-Cardassian Alliance.
The first known contact with the Terran Empire occurred when a transporter accident caused the landing party of the USS Enterprise to change places with their counterparts from the mirror universe, around stardate 3739. 
In this continuity Spock became the Empire's commander-in-chief as portrayed on DS9, but was overthrown by the Imperial Starfleet because of the influence the Halkans carried in his government. During a subsequent war with the Alliance, the Empire attempted to secure an Iconian gateway, which backfired when the planet sent up probes that attacked the ships' computers and disabled the invasion force. What the Empire had left wasn't enough to defend it, leading to its defeat.
As in The Final Days/The Gift Spock was overthrown because of the influence the Halkans carried in his government. David Marcus (mirror) attempted to carry on Spock's vision of reform through an underground resistance, but made a tragic miscalculation.
- As in the Dark Mirror timeline, this Terran Empire is from another parallel universe where they were never conquered by the Alliance.
Spock's new vision of reform only lasted until his own Captain Kirk returned to the mirror universe. The mirror Kirk, realizing he had been in another universe and the Empire could conquer that one as well, had Spock killed for his ideas and later became Emperor himself.
This particular version of the Terran Empire would not make anymore contact with their Federation counterparts until 2380 when Counselor Christine Nycz was sent on a mission to the other side aboard USS Alexandria to kidnap Lieutenant commander Dustin Zofchak for information he possessed about the transphasic torpedo. They wanted the transphasic torpedo so they could deal with their own Borg problem.
During Zofchak's torture interrogation, he coined the phrase "Mirror Universe" to describe the other side; the mirror Captain Allensworth even liked the phrase enough to start using it himself. ("Objects in the Rear View Mirror")
The Terran Empire started the war with the Dominion and took control of the Dominion. However, unlike the Federation, the Empire did not send Voyager into the badlands. As there was no reason to send them, Voyager remained in the Alpha Quadrant and there was no contact with any Delta Quadrant species other than the Borg.
With the transphasic torpedo in their hands, the Terran Empire took the fight to the Borg and eradicated the Borg Collective. Outposts were then set up in the Delta Quadrant to have a foothold in that area of space as well.
On stardate 57645.8, forces from the Terran Empire made their way into Federation space in the Talos star system. A battle ensued with both versions of the Alexandria going head to head. Due to the existence of the battle, the Mirror War between the Federation and the Terran Empire had officially begun. ("Advent")
The Empire gained its first foothold in Federation space by landing ground troops, led by Admiral Kathryn Janeway, on the Ba'ku (planet). The assault lasted a few weeks before they were thwarted back into their own universe. ("Requiem") Other planets the empire had occupied were Betazed ("Where Angels Fear to Tread") and Cestus III. ("Catalyst of Sorrows") An undercover Commander Merriell was able to get the defense codes and transmit them to Voyager, where they were able to defeat the imperial forces and push them off the planet.
The Empire then came up with the plan to kill Captain Kirk during the Khitomer crisis using the Guardian of Forever, but this plan was stopped when Captain Allensworth, Commander Merriell and Lieutenant Commander Zofchak went back as well and prevented the assassination. ("Held in the Hands of Forever")
The empire attacked Earth to find a fleet of Federation starships waiting for them, however the empire still had the advantage. That was, until the USS Alexandria-A showed up with more starships as well as Klingon and Romulan ships. Unknown to the empire, the Federation had sent four fleets to the mirror universe to attack the mirror Earth and take the Emperor and Empress prisoner. The Emperor accepted defeat and officially declared the Mirror War was now over and the Federation had won. ("Where Mortals Are Forbidden")
Some time in 2293, after the events of Khitomer Conference, the USS Enterprise-A encountered the mirror universe version of Khan Noonien Singh, "the most wanted man in the Terran Empire for most of it's history," who briefly mistakes them for their counterparts.
- Terran Empire article at Memory Alpha, the canon Star Trek wiki.
- Terran Empire article at Memory Beta, the non-canon Star Trek wiki.