Not a genre I've seen a whole lot in. Since you didn't specify any war(s) in particular, though, and using a very loose definition of a "war" film, I'd recommend:

All Quiet on the Western Front (1930)
Birth of a Nation (though if you don't think you could stomach the KKK as heroes, I'd pass on it)
The Bridge on the River Kwai
Casablanca
Casualties of War
Das Boot
Gone With the Wind
Little Big Man
Reds
Saving Private Ryan (first half, anyway)
To Have and Have Not

And, if you dare, Shoah, which is a 10-hour documentary about the Holocaust. I saw it many years ago at a campus theater, five hours each on two nights.

Reply to post #2 (show post):
Not exactly my "favorite" genre, either, but an important one ... In no particular order I'd agree on:

All Quiet on the Western Front (1930)
The Bridge on the River Kwai
Casablanca
Das Boot
Gone With the Wind
To Have and Have Not

I'd add, again in no particular order and applying the same loose definition of "war movie":

A Bridge Too Far
The Bridge at Remagen
The Longest Day
The Seventh Cross
The Judgment at Nuremberg
The Killing Fields
Good Morning, Vietnam
Gallipoli
Gettysburg
Gods and Generals
The Crossing (made for TV)

-- as well as, in terms of well-researched TV series:

The Civil War (Ken Burns -- nonfiction)
Holocaust (fiction -- series from the 1970s)
Foyle's War (fiction)

Apocalypse Now
Tora! Tora! Tora!
Schindler's List

... yes, of course!