I seriously believe the enemy is mainly going to be ONI aka Office on Naval Intelligence.Because they are known to set up at installations like these and known to sacrifice UNSC personnel.Think about it.It definitely shows in the books,games,and minifilms from waypoint.
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I would say your suggestion is plausible as well. We'll just have to see though, won't we? Honestly it could go either way. You made a good point. But either one could happen.