From Halopedia, the Halo wiki
Operation: SENTRY was an operation enacted by the United Nations Space Command between March 2555 and August 2558. Following the Human-Covenant War and the Battle of Tribute in the Epsilon Eridani system, remnants of the Covenant fleet remained in orbit over Tribute. Likely as part of the Unified Earth Government's efforts to recolonize Tribute, Region One of the UNSC's Central Command held negotiations with the Covenant fleet over Tribute. Olympia Vale served as an advisor to CENTCOM during this operation.
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