Following the fall of the Klingon-Cardassian Alliance, the Terrans started to regain power. Under the control of the mirror universe Julian Bashir they create the second Terran Empire in 2390. Bashir ruled the Empire until 2421, when he died. Over a hundred years later, in 2582, the Second Terran Empire controlled all on known space, but still had the desire to expand (the Empire's version of Manifest Destiny).
In 2590, the Empire discovers a reference to our universe, made during the Terran Rebellion against the Klingons and Cardassians, and planned to invade our universe. The planet of Andoria, homeworld to Andorians, protested and the emperor destroyed most of the Andorians. On September 10, 2602, the Mirror Universe sent the ISS Enterprise-O to our universe, where it was detected by the USS Enterprise-O. The ISS Enterprise destroyed the USS Enterprise, but not before Captain Aaron J. Riker sent a distress signal to Starfleet, warning of the attack.
With the area scouted, the rest of the mirror universe fleet enters our universe. Unknown to Starfleet, the mirror universe had cloaked ships, which slipped by Starfleet when they engaged the other half of the Terran fleet. They headed towards Vulcan,and attacked hoping to gain a foot hold in our universe.