The Forerunner-Flood War was a military engagement between the Forerunner species and the Flood that took place over approximately three-hundred years from 97,745 BCE until 97,445 BCE. The war was a continuation of a conflict from ten million years prior, the Forerunner-Precursor War, which resulted in the elimination of all Precursors from the Milky Way galaxy. Following their near-extinction, some Precursors converted themselves into a desiccated powder that, through a series of events and millions of years, became what is now known as the Flood. Though initially bent on total consumption of all sentient life, the Flood disappeared from the galaxy for thousands of years, seemingly under control of the Primordial.
They eventually returned, with a singular focus to wipe out the Forerunner civilization. The war raged for centuries until the tide was turned forever in the favor of the Flood when the Primordial infected the Forerunner AI Mendicant Bias with the logic plague, using his knowledge and skill against the Forerunners. Late in the war, the Didact was revived in part to help the war effort, but was largely unsuccessful. Though by far the most powerful and advanced society in the Milky Way, the Forerunners were almost completely exterminated, save for a very small handful who were able to fire the Senescent Halo Array from the Lesser Ark, triggering the Great Purification, and wiping out the Flood and all sentient species from the galaxy.