From Halopedia, the Halo wiki
Carson implied that she may have fought for the UNSC in some capacity during the Human-Covenant War, stating that they had "fought side by side against the Covenant" and that she had "seen Spartans in action firsthand".
After the Jiralhanae cruiser Foebane was destroyed during Operation: BUZZHAWK, Carson and her squad were tasked with securing the crash site. On their way to that location, they intercepted Gray Team and shot down a Banshee that had been strafing the Spartans. Carson was then ordered to disarm the Spartans and detain them for questioning, which Gray Team agreed to do. After the initial Sharquoi attack, Carson and her squad moved Gray Team to a bunker on the outskirts of the city where they were met by Ellis Gass and Lamar Edwards.
Carson and her squad were later deployed on a mission to kill the Jiralhanae Chieftain Hekabe alongside Gray Team, Rojka 'Kasaan, and a compliment of Sangheili warriors. They were tasked with defending the building that the Spartans had taken position on, doing so by mining the stairwell with explosives and positioning themselves on a landing above the building's lobby. After Hekabe was shot but did not lose control over the Sharquoi, Carson and her squad waited for a wounded Rojka 'Kasaan to pass their position towards the roof before blowing the mines, destroying the lower portion of the stairwell and trapping her squad on the landing. They were then engaged by three Sharquoi and fought them until their landing collapsed and they fell to the ground floor. Every member of Rae Carson's squad perished, either from the fall or being finished off by the Sharquoi. 
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