My observation came with a retired admiral.  The U.S. is the fastest military response world wide.  It's not about numbers, china does beat us there badly, it's not about weapons, because believe it or not the british have more of the fastest firing tanks, and strongest rifles currently made.  Most of the other nations training comes from the U.S.  And currently the U.S. Navy is the leading force on seas.  Marine Corps (with the help of the navy) Reacts just as fast and powerful on land. 

  The Retired Admiral was also a Skipper to a Medical Response ship.  He went on to some speach about a Tsunami that hit a small island.  And of course they where the first ones to responed, but close to 72 hours later, Germany came to help with that response.  And thou the admiral is retired, I believe he knows what he's talking about.

And yes the Army is in the Halo-Verse, and I think that the only thing that they have to help out with is thier research teams.  Army is well known for that, and in todays army, they are working on some serious super soldier armor and weapons.  They are also funded for rail-gun research which is something similar to the MAC gun.  I'm thinking one of the resonse why they are so small in the Halo-Verse (other than the fact that their is no ground for them to fight) is that they cut thier personnel for stronger research teams, and handed the rest to the Marine Corps and Navy Staff.  Their maybe some Army Soldiers left for guarding certain Military Installations.