From Halopedia, the Halo wiki
Operation: PHALANX 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 evacuation of civilians and observation of Covenant invasion patterns on the planet, utilizing intelligence gained from Operation: ABLE SENTRY's UNSC Marine force reconnaissance units. At least one Spartan-II—John-117—participated in the operation.
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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