As previously mentioned, "Das Boot" is a must see.
Also, there's another good one (It's probably already been listed above), I can't remember the name though. Not actually set in the theatre of war - but about the psycological effects of war, starring Bruce Dern as an officer who comes back home from Vietnam and totally cracks up. Wish I could remember the name! It features the song "Out of Time" by the Rolling Stones, and generally has an excellent soundtrack.
- found it on IMDB - "Coming Home" (1978)