Could agree with all of those selections but I'd add:
WWI: "All Quiet on the Western Front"
A fantastic movie, but I tried to stick with the American side of each conflict (that's why I didn't choose "Grand Illusion" for World War I). In fact, this was one of the hardest categories to choose from. Maybe a better list would be to choose the best films by war and by combatant. For example, World War I might look like this:
American: "The Long Parade"
British: can't think of one off hand
German: "All Quiet on the Western Front"
French: "Grand Illusion" or "Paths of Glory"
Russian: "Doctor Zhivago" (waiting for a movie version of Solzhenitsyn's "1914")
Italian: "A Farewell to Arms"
WWII: "The Night of the Shooting Stars" Gets into the idiocy factor of the impact of a war rather than the glorification. If it wasn't for George C. Scott's juggernaut performance, "Patton" is rather routine.
Well, I guess that's kinda' like saying that, apart from the scoring, Wayne Gretzky was only a routine hockey player.
Vietnam war: "Tigerland" Get into the roots of the futility of that war. "Apocalypse" unfortunately gets tangled up in some eerie satirical surrealism in the final reel that just leaves me cold. The documentary on making the film "Hearts of Darkness" is a better film.
I think "Apocalypse" gets into the psychological roots of the war. Plus it's a damn good film.
The original "The Alamo" was so flamboyantly out-of-sync with real history it's almost laughable and the new one goes the opposite way and at times becomes boring didactic exposition despite some good acting. I'm never negative about some extrapolations on unknown factors involving history but when it's so distorted its almost unrecognizable I can't quite swallow it.
I love the original "Alamo" precisely because it is so flamboyantly out of sync with reality. Besides, are there any films dealing with the Mexican War?