From Halopedia, the Halo wiki
During the Battle for Arcadia in 2531, Cargo 1, along with Cargo 2 and Cargo 3, were forced to evacuate thousands of civilians from the colony's besieged capital. The female pilot requested help after being attacked by Covenant forces. Despite several repeated attacks, the UNSC held off the Covenant long enough to allow Cargo 1 to escape the planet.
Cargo 1 is the primary target of Covenant forces after the destruction of Cargo 3.
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