A lot of my ideas have been posted already. That said:
"In the Hall of the Mountain King" by Edvard Grieg.
"Night on Bald Mountain" by Modest Mussorgsky (AKA the Devil's theme in Fantasia).
"Red Alert" from Star Trek: First Contact by Jerry and Joel Goldsmith.
"Imperial March" from The Empire Strikes Back by John Williams (I'm a little surprised nobody beat me to this one).
"Seven Seas of Rhye" by Queen.
"Immigrant Song" by Led Zeppelin.
"Rock the Casbah" by The Clash (the first song played by Armed Forces Radio during Desert Storm, incidentally).
"Lawyers, Guns, and Money" by Warren Zevon (good for mercenaries).
(Fun story: I once declared war on the local bass-thumper kiddies. I got a power inverter, put a 250-watt keyboard amp with a 14" woofer in the back seat of my Sentra, hooked it up to the Aux Out on my stereo, and put together a mix of "war music." So the next time a car pulled up next to me and started thumping, I cranked it up and blasted the Imperial March at 'em. The look on the kid's face was the best mix of contempt and respect I've ever seen.)
"Vox populi, vox humbug!"
- William Tecumseh Sherman