Reach is the next game in the Halo series. Halo: Reach is expected to cover the events preceding Halo: Combat Evolved, which took place on the planet Reach.
Within the Halo Universe, Reach was the last bastion of human military might outside of Earth itself. Its destruction at the hands of the covenant was a nearly fatal blow to humanity. Only the Master Chief's actions prevented the total annihilation of the human race.
For complete details about in in-universe world of Reach, find a copy of The Fall of Reach. In my opinion, the best of the Halo novels.
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