I have compiled a list of songs I consider anti-war songs. Many of them either state or imply that peace is preferable or even attainable:

At the Elbe, Fred Small                     
Ballad of the Soldier's Wife, Brecht/Weill
Band Played Waltzing Matilda, Eric Bogle
Before the War, Sydney Carter
Between the Wars, Bill Bragg
Christmas in the Trenches, John McCutcheon
Come Seamus and Ivy, Meg Davis
Come Away Melinda, Hellerman/Minkoff
Cranes Over Hiroshima, Fred Small      
Dancing At Whitsun, John Austin Marshall
Song about the saving of Denmark's Jews (title?)
Dona Nobis Pacem
Flowers of the Forest
From a Distance, Julie Gold
Give Peace a Chance, Lennon and McCartney
Green Fields of France, Eric Bogle
Green Grass of Shiloh, Beers Family
Hills of Shiloh, Shel Silverstein
Hoe, the Gun or the Guitar, The (author?)
House of Orange, Stan Rogers
I Ain't a-Marching Any More, Phil Ochs
I Come and Stand at Every Door (Dead Girl of Hiroshima) Nazim Hikmet
Journey, The, Linda Allen
Joy, Love and Peace
Kiri's Piano, James Keelaghan
Last Night I Had the Strangest Dream
Let Those Who Want to Fight, Fight
Let the Band Play Dixie
Mothers, Daightrs, Wives, Judy Small
Never Again Shall We Allow Another Atomic Bomb to Fall
No Mas
No More Vietnams, Fred Small
Pacem in Terres
Peacable Kingdom
Peace Call, Woody Guthrie
Peace Is, Fred Small
Peace Dragon, Fred Small
Peace Round, Jean Ritchie
Peace Will Come, Tom Paxton
Rosemary's Sister, Huw Williams
Study War No More
Take Off Your Clothes, Mark Levy
There Is More Peace Somewhere
Universsal Soldier, Buffy Sainte-Marie
Wall, The (Vietnam Memorial)
Walls and Windows, Judy Small/Pat Humphries
War, Chief Joseph
When Johnny Comes Marching Home
When the Lady Mary Sails
Where Have All the Flowers Gone?
With God On Our Side, Dylan