Specifically mentioning the Army, or themes about the army? Does it have to be the US Army? Does it have to be about modern conflict? Also, in one place you say military, in the other you say Army...

Since I expect you are looking for gung-ho, type music, I expect you will have difficulty. Reason being that almost all rock music about war & military themes is critical of war (even if it is sympathetic to the soldiers). You might have some luck with Death-Metal stuff, but it is not my area of expertise.

Still, here are a few off the top of my head

Matthew Good - Silent Army in the Trees. This one is genius, both poetically and musically. Definately worth a listen. I love how he contrasts the naieve idealism/imagination of our youth with the sometimes tragic realities of life as a soldier.

As for more well known artists:
Iron Maiden has a wide selection of tracks inspired by themes of War, "Aces High", "Where Eagles Dare", "These Colors Don't Run"
Metallica - Don't Tread on Me, Back to the Front, Disposable Heroes, Hero of the Day, One

Suggestions from others...
Ted Nugent apparently has lots of right-wing "patriotic" stuff, but I never listened to him, so I can't say.
Somebody suggested Godsmack, but I have no idea what song they are thinking of. "War and Peace" is about inner conflict, not war.