The Brutes have been fighting with the Elites for the Prophets' favour for a long time, according to Ghosts of Onyx.

And the Covenant had heard of the Flood before, they were mentioned in some holy Forerunner book or text, but they had no idea what it was until Halo.

I'd still buy Halo Wars even if it was just a UNSC campaign - as long as that campaign was awesome and not really short.