Not really a song I guess, but the Haka as performed by New Zealand's national Rugby team does it for me.

This version is from an Adidas commercial, but is great nevertheless.
http://www.youtube.com/watch?v=dJAjoCx6Euw

On a completely different level, I also like "De la Rey" by Bok van Blerk.
http://www.youtube.com/watch?v=vtKKJSfYraU

For at sea, "Salty dog" by Flogging Molly: http://www.youtube.com/watch?v=7NW29q6C4Do

With a song like this, you are invincible no matter the odds (at least feeling like it): "Fighting 69th", Dropkick Murphies: http://www.youtube.com/watch?v=9UmtJ30Rl5o

And what better then to sing about one of the most important things to make life worthwile: "Beer Beer" by Korpiklaani: http://www.youtube.com/watch?v=cXy6JHYa5Ks

And to throw the enemy completely off guard: http://www.youtube.com/watch?v=9AyotLiqJdg ;)

Well the Spartans used to have musicians accompany them as they marched into battle to give them a steadying effect and they even had their own war songs which I cannot pronounce let alone spell in the Greek.

From personal battle experience I couldn't hear a G..damned thing except for the ringing in my ears from exploding grenades, my M-60 and the ocassional RPG which exploded almost on top of me because those whom I was trying to dispatch found it annoying. I did hear some of my associates yell out "...that M.....F..... was close!" or scream "MEDIC!" Then there was the crack and pop that supersonic bullets make when they are coming very close indeed. I would venture to say that music and singing were the last things on anyone's mind. What we were thinking of was staying alive and sweet J.... get me through this just one more time.

Men of Harlech, preferably in welsh, preferably by a welsh coalminer's choir..... if not, then the bunch who sung in the Movie Zulu will do ok. JC