From Halopedia, the Halo wiki
In March 2555, a fleet of heavily armed Retrievers exited the Portal at Voi and began attacking the UNSC Home Fleet or mining Earth's surface. On the other side of the portal, a joint UNSC–Swords of Sanghelios team on Installation 00 managed to convince monitor 000 Tragic Solitude to call off the Retrievers. After the Retrievers retreated, UNSC administration decided that a fleet needed to be sent through the portal to investigate the source of the attack. UNSC Witness led a fleet of UNSC vessels through the portal and arrived at Installation 00. The carrier located the disabled Mayhem, the corvette used by the team, on the Ark's surface and the comparatively smaller Sangheili ship was lifted into Witness. After the corvette and its crew were stabilized within the carrier's cargo hold, Witness returned to Earth.
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