Ten million years ago, the Precursors, the species that populated the galaxy with intelligent life including Forerunners, determined the Forerunners were a failed experiment and decided to remove them from the Milky Way galaxy. The Forerunners, however, learned of this and decided that the only way they could survive was to destroy the Precursors, sparking the Forerunner-Precursor War. Very little is known about this war, though it is known that the Forerunners successfully eliminated the Precursors from both the Milky Way and Large Magellanic Cloud.
Following the war, the Forerunners removed all references to the event from their history, completely forgetting it ever happened. The Precursors, meanwhile, did many things to ensure survival, including converting themselves into a desiccated powder and sending the powder into deep space aboard space ships. This powder was later found by ancient humans and became the catalyst to what later became the Flood.