First Battle of Ares IV

The planet that would eventually be known as "Ares IV" by the UNSC was known to the Forerunners as "GA 44259-306". If they had a "proper" name for the planet, then it has not survived the millennia of abandonment, but it was colonised in approximately 98723 B.C.E., nearly six decades before the activation of the Halo Array. Ostensibly, the colony was to serve as a refugee camp for fleeing Forerunner civilians as the Flood consolidates its hold on the rest of the galaxy - in fact, the Forerunner known only as the Architect established the settlement primarily as an excuse to set up a weapons research facility on the planet separate from other Forerunner efforts, using the colony as a cover, part of his mysterious Labyrinth Array. As the Flood ravaged system after system, driving the Forerunners back behind the Maginot Line, Ares IV was left as one of the last free Forerunner colonies in the wider galaxy, and thus a prominent target for the Flood. While Mendicant Bias led a fleet to break through the Maginot Line and try to reach the Ark, another fleet entered the Mu Arae system.
Local defences were significant - the sheer number of colonists meant that removing them behind the Line would take time and resources the Forerunners could not spare, and their attention was already preoccupied by the needs of the Halo Array, but they had at least sent a small but powerful defensive fleet for orbital defence. As the Flood-infected warships, freighters, dropships and fighters entered the system, the Ares Defence Fleet engaged the enemy in space, virtually wiping them out. Unfortunately, nothing but total obliteration can stop the Flood, and less than a dozen singleships had managed to break through, landing on the planet, and quickly infecting the major population centres. Command of the battle was transferred from the fleet AI, LV-666 Curator, to the colony oversight AI 07/306 Overseer, and control of the planet's security and military forces was assumed. As the battle for the planet raged in earnest, the Overseer was forced to turn to the Curator one last time, to begin an orbital bombardment campaign.
Left with billions dead, but much of the Flood forces destroyed, the Overseer attempted to use the local Translocation Grid to transport the colonists to safety. Unfortunately, the Archivist, monitor of the local Labyrinth facility, deemed their continued existence an unacceptable threat to protocol - as long as they existed, the Flood would be able to multiply and overrun the planet. Their removal was swift, but brutal - Sentinels that had fought to save survivors from the Flood now turned on them as the Archivist overrode the authority of the Curator and the Overseer, seizing almost total control in a matter of seconds. Desperate, the Overseer managed to lock out several hundred constructs from the Archivists control, using a surprise strike by the Curator as a diversion, but by then it was too late - all but a handful of Forerunners had been slaughtered by the Archivist. These were escorted to the Translocation Grid, now locked out of the Archivists control to his detriment, and the survivors transported to safety.
Preparing to settle in for the duration of the battle, and expecting to eventually succumb to total Flood dominance, all three Constructs were surprised by the sudden and total collapse of the Flood as the Halo Array activated, sterilising the entire galaxy of sentient life. Realising that the colonists he had fought so hard for had died any way, and always would have died, the Overseer would fast succumb to rampancy, metastasising quickly, and contained by the Archivist. The Archivist would continue his research using specimens saved from the Halo's activation, spending the next 101217 years researching a way to defeat the Flood by other means. Lacking personnel to man its ships, the Curator settled its fleet on Ares IV's satellite, burying them underneath its surface and establishing a small repair and maintenance facility to keep the ships operational should they ever be needed again.
And as silence settled upon Ares, it lay dormant for one hundred millennia, until the arrival of the Reclaimers.