Jean Renoir's La Grande Illusion (1937)

Without any doubt at all the most powerful idea of the film comes after the announcement of the retaking of Douaumont by the French.  It's the singing of La Marseillaise by an English soldier who is dressed as a woman and who removes his wig.
Francois Truffaut, quoted in Renoir by Raymond Durgnat, University of California Press, 1974

Jean Renoir is something of a forgotten film director.  Watching La Grande Illusion recently on Turner Classic Movies I can see why:  the idea that everyone has their reasons is perhaps too subtle an understanding of humanity for today's audiences.  When I was in college I loved La Grande Illusion, somewhat mistakenly viewing it as mainly an anti-war film, the grand illusion being that a war could end future wars.  I found Renoir's La Regle du Jeu (Rules of the Game,1939) too complex in its exploration of class in France, not understanding that La Grande Illusion is more about class than it is about war:  German Commandant von Rauffenstein (beautifuly played by Erich Von Stroheim)has more in common with his aristocratic prisoner de Boildieu than de Boidieu does with his French colleagues Marechal (Jean Gabin), working-class, and Rosenthal (Marcel Dalio), Jewish. Rauffenstein shoots de Boildieu when he creates a diversion for the escape of Marechal and Rosenthal and is immensely sad that nationalities should trump class.  I still have the vivid memory, from when I first saw the film forty years ago, of Rauffenstein cutting his well-tended geranium to signify both the death of de Boildieu and the end of an era.

La Grande Illusion is beautifully photographed by Christian Matras, with a constantly moving camera that effectively captures the camaraderie of prison camps as well as the divisions caused by nationality and language:  when Marechal is moved from a prison camp he hurriedly tries to tell an English officer that an escape tunnel is almost finished but is unable to get through the language barrier.  But when Marechal and Rosenthal escape they are helped by a German woman running a farm with a very young son; she recites the battles where her husband and sons died and becomes, very briefly, Marechal's lover, even though she speaks only German and he speaks only French. The last shot of the film is a long shot of  the two escapees trudging through the snow to safety in Switzerland.