Throw in Patton, The Big Red One, the Longest Day, Sink the Bismarck, Courage Under Fire, Where Eagles Dare, U-571, A Bridge Too Far, and the Battle of Britain to the list above, and you're pretty well set.
Casualties of War is excellent, but it just happens during war, it's not really about war. I would say it's a better Vietnam flick than either Apocalypse Now (which is two hours of great and twenty minutes of flat-out weird) or the Deer Hunter.
I haven't seen Thirty Seconds Over Tokyo, but it always makes the critics' all time tops lists.
Other good ones are Judgement at Nuremburg, the Dirty Dozen, and Windtalkers.
Midway, Flight of the Intruder and Raid on Rommel are optional. I'd also go with the Hunt for Red October (not strictly a war movie) before I did Pearl Harbor.
Thirteen Days and the Great Santini are also very good, but again, not truly war movies.
Gods and Generals is decent, but can be very tedious if you're not a true Civil War buff.
The Band of Brothers miniseries from HBO (I think) is another excellent source.
