Operation: ABLE SENTRY
From Halopedia, the Halo wiki
Operation: ABLE SENTRY was an operation of the United Nations Space Command on Circinius IV during the Human-Covenant War at some point between 2531 and 2552. The operation saw the deployment of a unit of UNSC Marines—including Edward Buck—to provide force reconnaissance for Operation: PHALANX, an operation carried out by Spartan-IIs to evacuate civilians and observe Covenant invasion patterns on the planet.
In Halo 4: Forward Unto Dawn, John-117 tells the survivors of Hastati Squad that they are the only survivors on the planet. As the film is set in 2526, the presence of survivors years later ostensibly creates a contradiction. However, it can be assumed that Blue Team was operating on limited information at the time, or that the other survivors were only discovered later.
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