What makes Grand Illusion a great movie, and the reason that some of us keep returning to it, is that it can't be reduced to a single simple proposition, the way that recent war movies like Platoon ("war bad," to quote Tarantino's synopsis) or Saving Private Ryan ("war senseless") can. It's easy to be sentimental about war, even while deploring it, by focusing on the horror of it or by making heroes out of those who are forced to fight. Renoir deals instead with the far more complex mesh of differences and alliances that separate and divide our characters. And while his main characters all have a clear class/national/religious identity, he makes much more out of them than just sociological categories.
But trying to explain why Grand Illusion is such a great movie by charting all the conflicting bonds of nationality, class, religion, etc. doesn't explain why the movie is so powerful. To me it is in those scenes in which language either separates our characters (as when Marechal tries and fails to tell the British prisoners about the tunnel or asks why de Boeldieu uses "vous") or unites them (as when von Rauffenstein and de Boeldieu speak in English or the English officer (in drag) sings the Marseillaise or when Marechal finally learns a little German). In these cases, Renoir uses language-without hitting us over the head to make the point-to illustrate the conflict between his ideal of sympathy between humans and the differences of class, nationality and religion.
Now I know that this sounds just as dry and academic as other attempts to explain Grand Illusion. Maybe it is; the movie really does not need to be explained to be enjoyed. But these are the scenes that, for whatever reason, have always made the greatest impression on me.