I'll just list the ones that I feel might get overlooked.
"The Sorrow and the Pity" (French: Le Chagrin et la pitié)
If you like war history, this film is a must see!
"The Sorrow and the Pity" is a two-part documentary film by Marcel Ophüls that concerns the French Resistance and collaboration with the Vichy government and Nazi Germany during World War II. This 1969 film used interviews of a German officer, collaborators, and resistance fighters from Clermont-Ferrand. They comment on the nature and reasons for collaboration. The reasons include anti-Semitism, anglophobia, fear of Bolsheviks and Soviet invasion, the desire for power, and simple caution. The film shows the French people's response to occupation as heroic, pitiable, and monstrous, sometimes all at once. The post-war humiliation of the women who served (or were married to) Vichy men perhaps gave the strongest mix of all three. Maurice Chevalier's 'Sweepin' the Clouds Away' is the theme tune of the film.
"The Big Red One"
"A Bridge Too Far"
"M*A*S*H*"
"Catch 22"
"The Killing Fields"
"Barry Lyndon"
"Fail Safe"
"Guns of Navarrone"
"Force 10 From Navarrone"
"Manchurian Candidate" (Cold War)
"Battle Beyond The Stars" *Grin*
Or screw the movie idea, and watch the 6 episodes of "Black Adder Goes Forth". It takes place during World War One. That's about 3 hours.