Upon reaching Arcadia, two CovenantCCS-class Battlecruisers exited Slipspace. The Spirit of Fire arrived soon after. The UNSC Belfast, UNSC Texas, UNSC Pillar of Autumn, and UNSC Armstrong engaged the two ships immediately, catching one of them with its shields down and destroying it. However, the UNSC force suffered for its part in the battle. The Texas and Belfast were both taken down, the Armstrong was venting atmosphere due to damage, and the Autumn suffered severe damage before being assisted by the Spirit of Fire.
The second Covenant vessel was engaged in "surface activity" on Arcadia, attacking civilian targets and Pirth City, Arcadia's capital. The Arcadian Patrol, the main security personnel of Arcadia, was deployed to counter the Covenant invaders but, being a civilian force, was easily overwhelmed. SPARTAN Red Team and forces under the command of SergeantJohn Forge assisted in rescuing civilians, evacuating hundreds to several civilian transports abandoning the city. The Covenant, unfortunately, were successful in destroying several of these transports, resulting in massive civilian casualties. However, despite several repeated assaults, the UNSC was successful in saving at least a portion of the population before being forced to retreat to the city's outskirts, where they were reinforced by SPARTAN Team Omega. Forge's team fortified the outskirts and linked up with several groups of stragglers. These forces, led by the SPARTANs, teamed up to destroy a Covenant field base, from which the UNSC survivors suffered several assaults.
Following this, the UNSC forces established a base of operations at a deserted ONI base in the jungle. They used experimental Rhinoes to break through a massive dome-shaped Covenant energy barrier concealing several Forerunner ruins with technology the Covenant were using to upgrade an incomplete Super Scarab. The UNSC forces were constantly harassed by Covenant air patrols, but were successful in repelling them. After the shield dome was disabled, the Spirit of Fire barraged the area with several MAC round blasts then advanced on the Scarab's construction site, taking cover behind the Forerunner ruins as the Scarab fired on them. After a considerable effort on the part of the UNSC, they destroyed the Scarab and proceeded to investigate the site.
Despite the victory, the UNSC suffered a major setback as it investigated the Forerunner ruins on Arcadia. Under orders from the Prophet of Regret, the Arbiter traveled back to Arcadia and captured Ellen Anders, with the intention of using her to activate The Apex. The Spirit of Fire chased his ship to Shield 0459, unable to alert the UNSC of their target destination.
The UNSC regained control of the planet, but now that its location was known to the Covenant, most of its inhabitants abandoned the colony. A small number remained, establishing the settlement of Abaskun on the Mu continent, which supplied the UNSC with much-needed foodstuffs until the Covenant returned to the colony in 2549, glassing it.