Halo Wars isn't based on Contact Harvest. Actually, its pretty much right after Harvest. If you know the storyline (or don't, I won't try to give too much away), then you know the Covenant showed up at Harvest first. Whichever Admiral it was went and took a task force that outnumbered the Covenant 3 to 1. That was the naval battle that everyone knows but according to Halo Wars (you'll see it in the opening credits or so), it took humanity five years to take Harvest back. The Captain of Spirit of Fire makes that comment or so while he's looking down at Harvest after they have just retaken it. Then you continue along his story.
Suffice it to say, he finds Forerunner stuff... so a whole new story altogether.
Contact Harvest does not have any reference to Spirit of Fire. Basically its about how humanity met the Covenant and how the war started.
