Antiwar movies are actually one of the favorite themes in Hollywood movies. Since Hollywood isn't very spiritual, most of those kind of movies concentrate a lot on the butchery and dehumanization of the men. More than a few make sure that the movie is also a good war film to make sure has a good chance of doing well at the box office.
A lot of opposition to war is on spiritual and moral grounds, and a lot of films don't cover how men keep or regain their humanity during or after a war in the bigger spiritual picture.
An excellent movie on that theme is the 1954 Japanese masterpiece, "Harp of Burma," also known as "The Burmese Harp," directed by Kon Ichikawa, where a Japanese soldier who is being marched off as a POW sees the countless bodies of soldiers lying around in the jungle and becomes overcome with grief that virtually all of them will simply lie there until they rot, without their humanity preserved without even a burial.
He escapes and becomes a monk, and spends the rest of his life finding those bodies and giving them a proper burial.
The movie explores a few other themes, like one scene where the British soldiers trap a Japanese unit in a house, but the British know the war is over and the Japanese soldiers don't, so there's a tense moment where both sides are facing off, so the British commander orders his men to lower their weapons, and they begin to walk forward towards the Japanese soldiers singing auld lang syne, and the camera shows the soldiers faces and how worn out, fearful and tired they are of fighting, and after a moment of confusion the Japanese soldiers understand what's going on and lower their weapons and with the tension gone soldiers on both sides begin crying, it's a very poignant scene.
It's considered one of the best antiwar movies ever made, and it came out in a restored version a few years ago, which improved the movie as the cinematography was also masterful. It's a must for anyone who likes foreign films.
It was actually based on a novel, but this is one case where the movie version is better, if for no other reason the translation of the Japanese book doesn't bring out the nuances the movie did.
As it's a very spiritual story, there are scenes of warmth, and beauty, and one of the main themes is the harp which is the only possession the monk allows himself. In his ascetic life, and the misery and death that he sees, the music that he creates on the harp comes off as a metaphor for his soul, the warmth and beauty of which makes it possible for him to continue the melancholy task he's given himself for the rest of his life.
There's plenty of other classics and antiwar genre, and all worth seeing, and this is one film that belongs at or near the top of that group.