The mirror universe forces sent a cloaked fleet to Vulcan to attack it (and unknown to most people, to capture a hidden base on the planet). Several ships met the now uncloaked fleet in orbit over Vulcan. A few hours later, the USS Majestic showed up to help the fight. The battle was about to be lost when the fleet commanded by Frank Riker and William Deker appeared. When that fleet appeared, the mirror forces knew that they couldn't win, and retreated, getting a victory for Starfleet.