World War I - Paths of Glory (1957)
Lawrence of Arabia is a better movie, but Paths of Glory is a better WWI movie. That's how a lot of these things are gonna work. Not which movie is the best, but which movie captured the spirit of that war the best? In this case, WWI was insane, a folly so horrifically absurd and foolish it was supposed to be the last war ever. The movie illustrates this by presenting a story of three young men randomly picked for execution by firing squad for "cowardice" during an absolutely futile battle by a general who had inadvertently killed one of his own men the night before. Kirk Douglas stars as the colonel/lawyer who attempts to defend the men in an obviously fixed military tribunal. The end result leaves you feeling shocked and hopeless, like any good war movie should. And if you liked Stanley Kubrick before he got weird, this should definitely be your favorite movie of his.
World War II - Patton (1970)
If the famous opening speech written by Francis Ford Coppola and delivered by George C. Scott (and paraphrased from actual speeches by George Patton himself) doesn't epitomize everything that was wrong and right about this war, nothing does. Patton demanded nothing less from his troops than complete and utter domination, and while this method led to success on the battlefield, it became a zero sum game after the war ended and truces were to be made. That is the paradox of American military action in a nutshell. We can achieve victory, but we can't force the rest of the world to play nice when the fighting's over. Patton serves as an embodiment of this war, an old-school authoritarian who will not stand for any sissiness (publicly slapping a soldier of his who can't keep from breaking down) and who is already obsolete by the time the U.S. has won. That's the metaphor for how we treat our veterans: we take everything they have to give, and leave them with nothing.
Vietnam War - Apocalypse Now (1979)
Hallucinatory, flamboyant, violent, disturbing ridiculous and somehow beautiful -- in describing the film I could be describing the period of history that spawned it. Transposing the plot of Joseph Conrad's "Heart of Darkness" to Cambodia, this film's philosophy represents a shift from war movies of the past, from which cinema has never really recovered. Fine, we can win -- but only if we are willing to revert to our savage animal instincts and sink to doing absolutely whatever is necessary to gain the upper hand. In other words, if war is hell, it's our job to be the Devil. It's a dark, dark movie but an awesome one. I personally can't get through it more often than about once a year, but whenever I do it makes me wonder if this is actually the best film ever made.
Iraq War - The Hurt Locker (2009)
War is a game. That's the point we've come to. Patriotism is over as far as being a motive for fighting. The only thing left is personal victory. Winning doesn't mean anything anymore except on the absolute micro level. And the only way to win is to thwart every constant threat to your survival. And if those threats turn out to not be challenging enough, the whole thing gets rather boring. This movie is about what happens to one soldier, heading up a bomb-defusing team, when the boredom sets in for him. They say it's a movie that doesn't take any sides, but really it can be used to argue either way. It all depends what you bring to it. But the one place everyone who's seen it can agree is we're all lucky not to have to do this work ourselves.