Quite by accident, I caught the last half or so of the 1970 movie M*A*S*H this morning.  Ostensibly about the Korean War, it is really about Vietnam which, at the time of its release, was causing a growing backlash and protest in the U.S.  I was part of that protest.  The film was both a critical and commercial success, earning an Oscar for Ring Lardner, Jr. for his screenplay.
That same year produced the film Catch-22, based on Jos. Heller's great, great novel.  Although it garnered none of the plaudits or cash return of M*A*S*H, it eventually became a cult classic.  I love the flick, which has a wonderful cast including Alan Arkin as the protagonist Yosarrian.  Buck Henry's screenplay was criticized as being "disjointed," but I think it was a perfect re-creation of Heller's non-chronological book.
Then there was the 1964 Kubrick movie Dr. Strangelove or: How I Learned to Stop Worrying & Love the Bomb.  This remains a critically acclaimed film.
I find it interesting that all three of these great anti-war films are comedies, more specifically black comedies.  We laugh to keep from crying.  "Perhaps I know best why it is man alone who laughs," said Nietzsche.  "He alone suffers so deeply that he had to invent laughter."
There are a number of other great anti-war movies, but I only want to mention one because it is not generally regarded as such:  The Good, the Bad & the Ugly.  Because it was a so-called spaghetti western, it received a poor initial reception when it came out in 1966.  But it had several things going for it, not the least of which was the fabulous score by Ennio Morricone.  Quentin Tarantino has referred to it as the best film ever made.  The partial U.S. Civil War setting included a depiction of the two enemy sides, the North and the South, facing off over a bridge across a river.  The waste and horror of it are seen with all the fury that director Sergio Leone could muster.
Politicians get us into wars; film artists depict it not as glorious but as absolute slaughterhouse hell.  Yet the politicians persist.  You'd think that John McCain, one of our shrillist warmongers, would know better, having been locked up in a cage in Vietnam as he was.  But I cry when I see these films.  From the 1930 movie All Quiet on the Western Front, based upon the novel by Erich Maria Remarque, all the way to the more recent depiction of war horror Platoon, I watch them with tears and loathing.
"Human war," said Robert Ardrey, "has been the most successful of our cultural traditions."  So have anti-war films.