It is difficult to see why many people regard this as the greatest anti-war film ever made. Although its generally humanistic position (Renoir was explicitly adherent to Left politics during the 30s) on the stupidity of war in general is clear its terms are very much about WWI in particular. Whilst the number of casualties in WWII were much greater, WWI was so significant because of its cruel waste of human life with millions of anonymous soldiers dying pointlessly at the behest of the ruling classes of Europe in what proved to be their last hurrah.
The film shows none of the horrors but instead gives us a rather sportive view of the war (although not to the extent of Begnin'is Life Is Beautiful) as played out through the experiences of a group of French POWs and in particular, the relationship between two aristocrats, German commandant, Captain von Rauffenstein (Erich von Stroheim), and a senior French officer, Captain de Boeldieu (Pierre Fresnay), also a career military man. The film offers a classic "Prussian" performance from von Stroheim and although Renoir delivers the story with characteristically fine style, aided by the excellent photography of Christian Matras, it seems too staged to pack much of an emotional punch. Technically too it is dated with the camera relatively static and always simply set perpendicular to the action, giving the film a stilted quality something which also affects it dramatically particularly in comparison to more modern POW films such as The Great Escape (albeit that Sturges lifts one of Renoir's scenes holus-bolus)
In many ways the film's strongest card is represented by its title, which I assume refers to the illusion that the "Great" War was the "war to end all wars" (Renoir's film was made as the Spanish Civil War was erupting), itself only the most devastating instance of the human predilection for illusion in its myriad forms, from stage illusion to military etiquette, from the artifice of national borders to the illusion of love. It is this thoughtfulness which makes La Grande Illusion, despite its datedness, still worth watching.