Journey's End
There have been quite a few movies about the Great War and the trenches in particular. I can't claim to have seen them all but I've seen a few, and this one is a stand out I think. It let's us see how the spirit could be crushed in some people but shine through in others.
I just looked it up and it's in fact the 5th screen adaptation of a 1928 play.
Highly recommend.
