Just speculating so please don't flame me.
But theres all these theories going around, some including Masterchief going back in time, which would probably lead to a paradox.... Unless!
What if mastercheif was actually sent back in time through an alternative universe, think of all the possibilites. Maybe the enemy is differant? maybe nothings the same? An altarnitive universe, an altarnitive reach?
*It has been said there are more then one halo universe...
[Edited on 06.21.2009 3:20 PM PDT]