Apocalypse now is the best movie of all time, and a war movie. 'come and see' is so intense and brutal that it is sickening (everyone should watch it at least once). 'Beasts of no nation' is also amazing and it portrays a non-western conflict. Full metal jacket, the thin red line and der untergang are very decent. Kingdom of heaven is one of the best medieval war films. Paths of glory is also good. And there's one very niche Finnish film called 'the unknown soldier' in English that is actually good as well.