Very good idea for a thread.
'King and Country' - and underrated - and sobering - British movie from the mid sixties directed by the late, great Joseph Losey, (and featuring Tom Courtenay and Dirk Bogarde among others).
The adaptation (but characteristically, while actually rather good, not anything like as good as the original, superlative book by the excellent Pat Barker on which it was based), of 'Regeneration'.
Dr Strangelove (obviously - sheer class - and utterly brilliant).
Paths of Glory, certainly, excellent and intelligent movie.
I agree with the inclusion of MASH as an anti-war movie, but I don't think Das Boot (while an excellent movie) is a classical anti-war movie, as much as it is an attempt to show that not all Germans were fanatical Nazis, and that one could be a brave, patriotic, if clear-eyed soldier without necessarily supporting the war aims of own's own (loathsome) Government.
Catch-22 certainly; and, again, another case where the book it was based on is light years better than the movie.
The Accident - a thoughtful and very well-made Dutch movie (based on an excellent book of the same title by the Dutch writer Harry Mulisch).
The Life and Death of Colonel Blimp - a lovely, thoughtful, intelligent, nuanced British movie made during the Second World War; Winston Churchill loathed it, apparently, and attempted (unsuccessfully) to have it banned.
There are others which come to mind....Apocalypse Now, yes, most certainly, I'd include that on a list of anti-war movies, but I'd argue that Saving Private Ryan, and Avatar belong to other categories.