I'm gonna go ahead and recommend the most horror, non-horror movie ever; Come and See 1985 directed by Elem Klimov. It's a russian film about WW2 and it is told through the perspective of a little boy. He wants to fight agains the nazis even though his mother forbids him but eventually gets taken anyway by soviet soldiers. Its extreme realism, even using real guns for most scenes and horror sound design give the movie its raw quality. It is the most horrifying film I have ever seen and I have seen some weird shit.

The cinematography is absolutely amazing and at the end has one of the best montages in cinema. Might even be the best montage because I can't think of anything that tops it. You can see what WW2 does to people and how it went on. Some terrible shit happens in the movie and the director himself even said that wasn't the worst he could've shown. Imagine that. I cried a bunch of times and was very depressed after the film was over because I had to live in a world like this. (though those feelings will flow over, no worries)

At the end of the day this is probably a once in a lifetime watch, or maybe watch every year or so. If you want to experience a war film you have never experienced like any other war film before then I will say: Watch it.