Ensemble, the makers of Age of Empires 3, has been asked by microsoft to make a Halo RTS, named Halo Wars. Microsoft says that it’s only for the Xbox 360 and will not be for the PC.
The story can be viewed here as a movie (warning this is the high quality movie so don’t view if your internet sucks):
According to MS, the game is before the original Halo, so around the time when the UNSC Marine forces were first encountering the Covenant.
So it’s a big fat war game. Check out their website here: