Forerunner-Flood war

The Forerunner-Flood War 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. After 300 years of devastating warfare between the Flood and the Forerunners, the highest levels of Forerunner Fleet Command began to realize that Forerunner species extinction was possible, as so many Forerunners had fallen to the Flood. After Flood spreading continued, planetary self-bombardment after Flood infestation turned into complete system-wide destruction by detonating planetary system stars after even minimal Flood presence was detected in a Forerunner system. Forerunner military forces were ordered to don heavy armor and other personnel were ordered into protective stasis.

In their own words, hundreds of unsuccessful methods attempted, and all possibilities were exhausted to stem the tide of the Flood before the Halos were fired,. The willingness of the Forerunners to essentially slit their own throats in order to stop the Flood attests to both the futility of preventing the Flood by more conventional methods, and the horrors of the galaxy enthralled to the Flood. In defense of the Flood, the Forerunners built at least two shelters, the Ark and the Shield World, for use when the Halos had fired.

At least in the last few hundred years before the conclusion of the War, there were Forerunners cataloging sentient species (the Forerunner are known to be familiar with the Sangheili and San 'Shyuum species), and with very little time left, the human species was discovered.