Well there is possablity that Halo 4 may explain more about the The Forerunner-Precursor War. Not only the War it's self but also the last surviving Precursor, the Prisoner of Charum Hakkor who is known as the Timeless One, Who seem to know's more about what the flood truly are.
(It has been a while since I read any of the Halo History so I could be wrong.)
There is an official story synopses for Halo Trilogy on Xbox.com.
http://www.xbox.com/en-US/Marketplace/Product/Halo-4
The site says
Wake up, John.
Halo 4 is the next blockbuster installment in the iconic franchise that's shaped entertainment history and defined a decade of gaming. Set in the aftermath of Halo 3, Master Chief returns to confront his own destiny and face an ancient evil that threatens the fate of the entire universe. Halo 4 marks the start of a new trilogy that begins with its release in 2012.