Sorry to burst alot of bubbles but the FAQ says it all:
Q: Where does “Halo Wars” take place in the Halo timeline? A: “Halo Wars” takes place before Halo 1 during the UNSC’s first deadly encounters with the Covenant.
Q: Can you reveal anything about “Halo Wars” storyline? A: Players will lead the crew of the UNSC’s “Spirit of Fire” from initial skirmishes to an all out war against the evil Covenant.
Q: Will I be able to play as the Covenant? A: Stay tuned.
That pretty much kills any notion of playing on Reach, or Installation 04. Also without being able to play as the covenant (yet), we're likely restricted to the few battles the humans actually do win, otherwise risking a compromise of the storylines.
And I'm sorry "christian" but:
Q: Will I be able to play Halo Wars on Xbox 360 and PC? A: Halo Wars is for the Xbox 360 exclusively.