You don't have faith in the Marines to defend themselves?
If they have insufficient training and lack of access to their weapons (or ordered to hold their fire) due to illegals crossing, then yes, the situation could get ugly. I'm not saying the Marines there can't defend themselves, but they'll be pretty much sitting ducks.
Not if they are students, walking between the barracks and the chow hall, with their weapons locked up in the arms room...as is generally the case.
The article is about illegals on the ranges - which happens all the time in Arizona. The terrorist scenario would have them going to the cantonement area to wreak their havoc.