From Halopedia, the Halo wiki
Circinius IV[note 1] was a Unified Earth Government colony world in the Circinius system. The planet was orbited by at least three natural satellites. It was the location of the Corbulo Academy of Military Science, an officer training school for the offspring of high-ranking UNSC officials. The space elevator located at Corbulo Academy was considered one of the planet's most spectacular sites.
After the first engagements on Harvest, ONI Section II kept the Covenant invasion secret from the general population. However, Corbulo Academy's superintendent, General Daniel Black, was informed. As preparation for a possible attack, Black discreetly increased the readiness of the UNSC Marine garrison posted in the academy and authorized limited communications blackouts under the guise of routine maintenance procedures.
The colony and the academy were attacked by the Covenant during the earliest days of the Human-Covenant War on April 26, 2526. Orbiting UNSC vessels soon deployed Orbital Drop Shock Troopers directly onto Corbulo Academy's campus. The planet's space elevator was also destroyed, killing many of the escaping cadets as it fell. During the battle, the Covenant killed almost the entire population of the planet. Only four cadets from Corbulo Academy survived the initial assault, although one of them, Chyler Silva, died during the battle. Later, the planet was glassed by the Covenant.
At some point following the planet's invasion, the UNSC launched Operation: ABLE SENTRY on Circinius IV to investigate Covenant activity on the planet. Edward Buck participated in this operation. Directly afterwards, Operation: PHALANX was initiated and carried out by UNSC personnel, including John-117, to rescue civilians on Circinius IV and to investigate Covenant invasion patterns.
Behind the scenes
Along with Hat Yai, Circinius IV is not mentioned in the Assembly's report on glassed worlds (dated January 1, 2530) in data pad 14 from Halo: Reach. From a real-world perspective, the obvious explanation is that the planet was created after the game's release, there is no clear in-universe reason for this omission. However, given that both worlds had some form of human population on their surface following their presumed glassing dates, it is possible that both Circinius IV and Hat Yai were not entirely glassed by the Covenant.
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