I'm bemused by the tendency to assert that this film is a veiled Vietnam film, "ostensibly" about another war, as this Amazon review puts it. It IS about another war, one that was far more important to the nation where it was written and filmed, and in which the father of writer and director Charles Chilton was killed. Anyway, I watched it as a First World War film, and that is how it has always seemed to me: it comments on all other wars, implicitly, of course.
It's silly to compare or measure this against most other war films, because it is so unlike any other, but it stands out as a dramatic, cinematic, narrative gem. A serious musical about the horrors of war: sounds as likely as a serious musical about living in Nazi Germany. Oh, wait...someone did that too. Cabaret must owe quite a bit to this film, not least in the "tomorrow belongs to me" scene, although they wrote their songs from scratch for Cabaret.
The songs here are real, some the official versions from popular music hall, and some the unofficial versions sung by the troops, with considerably darker lyrics (though they omitted the rudest of the unofficial lyrics). The humour is black and dry as a tomb, and you don't quite know whether to laugh or wince in a lot of places (just do both). But the real beauty of the film is in the settings, which are sparse, only partly realistic, and sometimes subject to extraordinary changes. The most impressive are slow 360 degree pans, during which everything changes behind the camera's back, so that when you get back the character you started with, they are in a completely different situation. These and other rapid scene shifts are part of whole film's unreal, nightmarish quality that matches the subject matter perfectly.
If you haven't seen it, make sure you do. If you saw it long ago and dimly remember it and wonder if it was as good as you remember (or maybe better than you thought), I'd say yes, and you should refresh your acquaintance. This seems an almost absurdly cheap price for it.