hey dipwad, take 5 seconds of your life to confirm facts before sounding like a total ******.
So here's a brief rundown of the war(in bad grammar).
100,000 years ago
Flood attack human and san shyumm (prophets).
Human & prophet form alliance and in other escape flood evacuate their infected planets and take over planets occupied by lower species.
Forerunners consider this an affront to mantle (they unaware of flood) and start war with human-prophet alliance.
Human-prophet find a cure to flood, flood driven off, but the alliance gets destroyed by forerunners, cure to flood lost.
Flood comes back and proceeds to rape forerunners. Forerunners destroy all life including themselves.
The end.