The [i]Infinity[/i] is the flagship of the UNSC Fleet. I think it stands to reason that there are officers and non-officers of every major branch of the Military aboard. Air Force - Pilots. maybe? (Granted we see no Hawks or Hornets, just Pelicans, and you could easily have Marine pilots) Marines - Obviously infantry. Army - Not every soldier is cut out to be a Marine, but they should still have combat experience with the Covenant/ Prometheans. Navy - It's a ship. It's manned by Navy personnel. Spartans seem to be their own division of the Military now. Kind of odd that ONI would relinquish command of the UNSC's most important program.