No one mentioned:
Games Without Frontiers - Peter Gabriel?
The track The Bravery Of Being Out Of Range on Roger Waters’ solo album Amused To Death is a solid war song with that’s both musically and lyrically competent.
Others have mention Pink Floyd’s When The Tigers Broke Free, a deeply moving war song. As for The Dogs Of War, I believe it to be one of Pink Floyd’s worst, and lyrically trite. Say what you want about Waters’ ego, but he knew how to write lyrics that leapt right out at you. Also, he’s more qualified than most singers to comment on war - his pacifist father was killed in action in Anzio during WWII. Most of the Floyd album The Final Cut is about war (heck, it was subtitled A Requiem for the Post War Dream).
**ME 262 **by Blue Öyster Cult. I love the lines:
How about “Gettysburg”, “The Seige” and “Warriors Son” by The Brandos.
Pick up the original sound track for Iron Eagle
Declan
Another vote for Heather Alexander’s March of Cambredth.
Two Brothers (On Their Way) - not sure who wrote or performed it
Roll Alabama, Roll - trad.
Powder Monkey by Schooner Fare
My Youngest Son Came Home Today by Mary Black (I believe it’s a modern adaptation of a much older song - hers is very clearly about modern Ireland. A similar song is My Son John from Boiled in Lead, which is a more general gunpowder/Napoleonic era song.)
Crusader by Chris deBurgh
Prince of Darkness by Indigo Girls (Not exactly a war song, but it often ends up on my martial playlists.)
The Grave by Don McLean
Sending the Old Man Home by Jimmy Buffett
The Grey Funnel Line, trad. I’ve got a copy from Silly Sisters. Not precisely a war song, but a song about life in the Royal Navy, after they went to steam - and I think dating to the post-Dreadnought era.
and, of course, you need to end with a bagpipe version of Amazing Grace.
Oh, don’t forget Prokofiev’s Alexander Nevsky suite. Another great classical war piece.
Wow, how did I forget that. Must be included!
Steve Earle “Rich Man’s War” 
Someone already mentioned everything off of War, by U2. Some more U2 would be:
Bullet the Blue Sky
Love and Peace or Else
Love is Blindness (anti-nuclear war)
"Danny Boy" could fit, I suppose, since it seems to be sung by a parent sending a son off to war (“the pipes, the pipes are calling”).