Forerunner-Flood War

The Forerunner War was a conflict between the Forerunners and the Reaper-Flood Alliance that occurred milions of years ago, before the founding of The League.

It started when the idiotic Forerunners accidently unleashed the Flood by attempting to bully them, and as a result it started to attack them. Just as that battle was beginning, the Reapers returned for their usual cleansing that they undertake every 50,000 years.

There was no such formal declaration of an alliance between the two species, but when the Flood advanced to the co-ordinated stage, they were under the leadership of Gravespeare, who decided to talk to Harbinger. The two leaders decided that their mutual goal of destroying all life would be achieved quicker if they co-operated.

The Forerunners, being idiotic failures, lost battle after battle. As Reaper ships decimated the Forerunner fleet and Flood demolished Forerunner ground forces, the stupid Forerunners could do nothing. The almighty Didact led his apparently invincible fleet and got demolished. But they came up with a plan. Instead of just blowing up their enemies, they decided to blow up everyone in the Galaxy. Good Idea. Even Reapers weren't that bad, they wiped out all advanced life and left younger species alone to grow. But no, these idiots thought blowing up everyone was the only way.

So they done it, they blew up everyone. Except their enemies. Since Halo rings have a specific range, Gravespeare and millions of Reapers were able to retreat to Dark Space and escape the blast, ensuring their survival. Granted, all of the other Flood were wiped out, but the Forerunners failed in their goal of destroying their enemies and instead just destroyed themselves.

At least one of them, The Librarian, had some degree of intellect. She decided to keep specimens of each species and use these to repopulate the Galaxy. It is thanks to her that civilization as we know it exists today, and why the stupid Forerunners didn't doom the Galaxy.