Two words: “Halo War”. As seen from the developer’s web site:
“Halo Wars” is the next big real-time strategy game from Ensemble Studios, creators of Age of Empires. Designed exclusively for the Xbox 360, “Halo Wars” allows you to take charge of the human UNSC armies and see the Halo universe from a totally new perspective. “Halo Wars” takes place before Halo 1 during the UNSC’s first deadly encounters with the Covenant. Players will lead the crew of the UNSC’s “Spirit of Fire” from initial skirmishes to an all out war against the evil Covenant.
A release date has not been set. Being for Xbox 360 only sucks big time. Meantime, you can check out images or download a 720p 5.1 surround sound version of the trailer here.