Some of my favourites have already been mentioned, so I’ll go a bit further afield and mention “Attack in the Pacific”. It’s made in a very sober, serious style, without the propaganda hysterics of early war films, and it devotes a lot of time to what goes into the planning and execution of an amphibious assault.
Note by the way that the dates cited for this film are misleading: the Internet Movie Database says it was made in 1944, and the film itself carries the date of (I think) 1950. Both are wrong. It’s clear from watching the film that it was made after the invasion of Okinawa (which is mentioned) but before the atom-bombing of Japan (which isn’t mentioned) or the Japanese surrender (because it’s evident from the closing narration that the war is still going on). My guess is that the war ended before the film could be released and that it was shelved until 1950.