GameTrailers TV got its hands on the backstory trailer for Halo Wars. The 3:28 video sets up what's going on and why in the forthcoming RTS based on the Halo universe. Seems the Covenant have been scraping around in the ice on some planet and have found something that, of course, "it could just be the key to the whole war." No gameplay, but a lot of threatening mandible-flexing.
Halo Wars - Field Trip to Harvest [Gametrailers]