Le Grande Illusion has apparently been called "The Greatest Movie of all Time", while I can see where that comes from, I don't completely agree. It is a great movie and the director, Jean Renoir (who has an awesome name) made quite an interesting movie using depth of field, an interesting storyline and location shots.
During World War I two pilots are shot down over German enemy territory and are taken to meet a German officer, von Rauffenstein, who welcomes them to lunch where they talk and are quite friendly. They are then taken to a POW camp where they live less like what you'd assume a POW camp to be like and more like they are just at a camp they can't leave. While they plot an escape via a tunnel they also put together a musical show. During the show they learn that Douamont has been taken back by the French, which earns one man, Marechal, a trip to solitary confinement.
After several more escape attempts they are moved to a POW camp that is notoriously impossible to escape from and meet back up with von Rauffenstein. By this time von Rauffenstein has seen a lot of battle and is quite a mess. He remembers the two men from before and becomes close with de Boeldieu, a fellow aristocrat that comes from a well-known family, and they strike up a friendship of sorts.
Unlike most "war movies", this movie is more about the characters and what it means to be human during war and about how class lines disappear during a war. It also features dialogue in three languages: English, at times, French and German. It's quite an interesting way to present other people and cultures.
I highly recommend this movie and this is why: If it was in color and released today, you would never know that it was made in 1937. It's made that well and it is highly enjoyable.