A Halo strategy game
Fair enough. I will admit it's a bit of an awkward zone for roleplaying as opposed to something more down the general wargaming/strategy line, because most of the systems I'm familiar with are more linked to individuals on a relatively personal level, as opposed to a overarching planet or galactic level. It might be worth looking at general wargaming systems when you're looking at systems to get a starting point, as long as they're abstract enough to translate to forum play.
I know people have run faction level games on Rpol before, but I will admit I don't know exactly what they used to show that beyond homebrewing or basing on something found in strategy or turn based video games, or board games such as Risk.
Alternatively, if you're more interested in the drama and interpersonal conflict of co-ordinating such an assault rather than actually the nitty gritty of dice rolling against a GM controlled enemy, I suppose you could include freeform, although you'd probably want to emphasise to your players that they had to bear in minds reasonable limits for their forces and what they can achieve.
You might have some luck if you try and search the Players Wanted thread using keywords like strategy, faction or something similar, and seeing if the results yield anything which is similar to what you're looking for, and then RMail the GMs to see if they can give you advice. Apologies that I can't give you more concrete advice!