The Battle of Meridian was a three-year conflict between the Covenant and UNSC forces on and around the colony world of Meridian, a moon around Hestia V in the Hestia system, between 2548 and 2551. While the sizable UNSC forces in the system were able to hold off the Covenant advances for years, they were eventually forced to retreat and the world was glassed in 2551.
After the moon was glassed, the Covenant forces were able to recover a Forerunner Luminary from the moon's surface. This luminary eventually disclosed the location of the portal to the Ark to the Prophet of Regret, who amassed a fleet to investigate. Unknown to Regret was that the portal was located on the human home world, Earth. This began the Battle for Earth.