From Halopedia, the Halo wiki
The facility was constructed at the base of an ancient Forerunner structure recently discovered by the UNSC. When it became clear the Covenant were heading for the Forerunner structure, UNSC command feared Alpha facility would be crushed. Sometime in 2554 the facility was attacked by Covenant forces. Sarah Palmer, with a Marine detachment, infiltrated the Covenant artillery positions and destroyed them. Edward Davis noticed the Covenant's heavy reliance on dropship insertion. Spartan Davis realized they would be vulnerable to focused anti-aircraft fire, so, using M9 Wolverines, he began establishing air defenses west of the facility at a location designated Glacial Perch.
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