Forerunner-Flood war
From Halopedia, the Halo wiki
The Forerunner-Flood War[1] was a galactic war between the Forerunners and the Flood.
The Flood entered the Milky Way galaxy at least at 98,379 BC, and initially caught the Forerunner military by surprise, using captured non-military vessels to penetrate local Forerunner naval fleets and descend and land upon Forerunner-colonized worlds, overrunning local defenses and trillions of Forerunners per world with billions of Flood forms within days. Forerunner fleets were forced to bombard Flood-infested Forerunner worlds to prevent Flood spread. The Forerunners grouped together under two centralized governments: first the Mantle and afterwards, the Guardianship, and bombarded dozens of Flood-infested former Forerunner planets.
It ended in 97,448 BC with the firing of the Halo Installation and the believed death of all the Forerunners.
