From Ensemble Studios' own web page is this announcement for Halo Wars, which may assuage some of those already screaming that the meagre amount shown in the trailer contradicts known canon:

Breaking News - Ensemble Studios is bringing real-time-strategy to the Xbox 360 and to the Halo universe. 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 is set during the UNSC's first deadly encounters with the Covenant, allowing players to lead the crew of the UNSC's "Spirit of Fire" from initial skirmishes to an all out war against the evil Covenant.

It's been pointed out that the very first Covenant-Human encounter was at Harvest, when no Spartans were present. However, I certainly think there are enough gaps in the record of the Human-Covenant war as presented in the novels to place a ground battle on a human colony world with Forerunner artifacts the Covenant want access to, where Spartans are in play and the Covenant are not yet glassing all Human worlds as a matter of policy, or are perhaps avoiding glassing one in particular for specific reasons.

Everybody calm down.