It's the Lucas happy-go-lucky BS that they won. In any case, the galaxy was better off with the Empire. When it collapsed (for lack of leadership, which I also find BS (no contingency plans!!!!!!1!!11!one!)), a power vacuum emerged and the galaxy went to hell like Somalia in 1991.
I assume this would be a battle against the modern day humans of the Halo universe? Because if it was a battle against the Forerunners it would be pretty one sided. Or heck if it was a battle against the Precursor's it would be a flat out genocide.
On a side note, I'm an Imperial man. I've taken a look at both ideologies and the Empire's base cores aren't bad at all.