Let us not for get that this is the UNSC (United Nations Space Command)  these are mixed units of different nations combined into a crazy military.  Of course there will be a little strange referance to other military personnel,  and knowing that the U.S. is currently the top military with the british not far along with the Chinese.   In Halo--verse,  The Navy/ Marine Corps are both the dominant force, much like said above...  The Space is the navy's seas, and the planets, are the marines play ground.  Army is still around because they have the best research funds to make the war better.  ONI (office of naval Intelegance) may still be used often in the books but that's because they where under the Spartan project, and the best for Intellegance gathering. 

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