In April 2063 a Borg sphere travelled back in time to stop Zefram Cochrane's first warp flight, and the Enterprise-E followed it to preserve history.
After its defeat at Earth in 2373, a Borg sphere escaped through a temporal vortex to the day before first contact, intending to assimilate an Earth still devastated by World War III. The Enterprise-E followed, destroyed the sphere, and found that it had already bombarded Cochrane's missile complex near Bozeman, Montana.
Picard's crew worked with Cochrane and Lily Sloane to repair the Phoenix, while Borg drones who had beamed aboard the starship assimilated its lower decks. The Borg Queen captured Data and tried to win him over, and the drones tried to signal the Borg of that era from the ship's deflector dish.
Cochrane made his warp flight on 5 April 2063 with Riker and La Forge aboard, and a Vulcan ship arrived as history recorded. Data then released coolant that destroyed the Queen. Debris from the sphere remained in the Arctic until 2153.