Alpha Base was the name of a UNSC military base located in the northern polar regions of Harvest during the Harvest Campaign. Overrun by the Covenant at some point, it was later reinforced by the UNSC Spirit of Fire in February 4th, 2531 with the help of Sergeant John Forge.
The base was in close proximity of the Forerunner Relic site, and was used as a base of operations in the UNSC operation to assault the structure. After being reinforced by the Spirit of Fire, the UNSC forces managed to breach the Covenant defenses around the Forerunner complex and access it. The base was likely abandoned after the Spirit of Fire left the planet.