Since I'm half Irish (other half blarney), I usually turn to the Irish ballads in times of anti war songs.
I've always liked the simplicity of John Lennon's 2 chord song. Once the Smothers Brothers were talking about being up on the balcony with John leading the crowd in this song, and Tommy was doing all kinds of creative versions of a D chord, and John mildly chastised him saying just play a regular D chord, man, this is about something very basic & simple:

All we are saying, is "give peace a chance." etc.


Here's one sung by Peter, Paul & Mary:

CRUEL WAR

The Cruel War is raging, Johnny has to fight
I want to be with him from morning to night.
I want to be with him, it grieves my heart so,
Won't you let me go with you?
No, my love, no.

Tomorrow is Sunday, Monday is the day
that your Captain will call you and you must obey.
Your captain will call you it grieves my heart so,
Won't you let me go with you?
No, my love, no.

I'll tie back my hair, men's clothing I'll put on,
I'll pass as your comrade, as we march along.
I'll pass as your comrade, no one will ever know.
Won't you let me go with you?
No, my love, no.

Oh Johnny, oh Johnny, I fear you are unkind
I love you far better than all of mankind.
I love you far better than words can e're express
Won't you let me go with you?
Yes, my love, yes.

Yes, My Love, Yes.


And then there is the ever popular:


Look At The Coffin

Look at the coffin with its golden handles.
Isn't it grand, boys, to be bloody well dead.

chorus:
Let's not have a sniffel
Let's have a bloody good cry.
And always remember the longer you live,
the sooner you'll bloody well die.

Look at the flowers, all blody withered,
Isn't it grand boys, to be bloody well dead.

chorus

Look at the mourners, bloody great hypocrites,
isn't it grand boys to be bloody well dead.

chorus

Look at the preacher, bloody santimonious,
Isn't it grand boys to be bloody well dead.

chorus

They sing about glory, and honor the war,
Isn't it grand boys to be bloody well dead.

chorus