Found some evidence that it may be a prequel: Look at the right corner at 0:18. It says '2552', and Halo 3 is set in 2553. It could be a 'mid-quel' of sorts, actually, since I believe Halo 1 was set sometime in 2552 (though there is only a difference of a few weeks between the three games).
So, it could be something going on at the same time as Halo 1, after it, or earlier in the year.
This is all assuming that the '2552' means anything.







