The first World War and several French officers, among them Lt. Maréchal (Jean Gabin), Capt. de Boeldieu (Pierre Fresnay), and Lt. Rosenthal (Marcel Dalio), have been captured by German forces and escorted to a prison camp where they quickly go about the business of planning an escape.
The obvious choice, seeing as how the Germans have provided rudimentary gardening tools, is to tunnel for freedom, but the day before the tunnel is completed, the prisoners are rotated and our three heroes moved to their new home--an old Bavarian fortress, high and impregnable.
It is here that Boeldieu is reunited with Capt. von Rauffenstein (Erich von Stroheim), a pre-war acquaintance who laments that a man with such high breeding as Boeldieu should be subjected to such conditions. Rauffenstein is a kind warden but, as Boeldieu says "prisons are made for escaping," so they set about planning their early release.
Even if Jean Renoir's La Grande illusion is not the first anti-war film ever made, it is certainly one of the best for the simple fact that it assumes we understand how awful war is. There is no gruesome battle footage, no stirring calls for peace, and no indication that anyone in the film thinks the war to be a remotely good idea.
But both sides accept it as something they must endure, much in the same way we view traffic jams--as a nuisance beyond anyone's control. When they talk of reaching neutral Switzerland there are no dreams of hiding in a small village until the fighting ends. They speak almost reluctantly of rejoining the fight, not because of any intrinsic desire to shed blood or an overt sense of nationalism, but simply because they haven't allowed themselves to consider an alternative. To stop fighting because they've escaped is to them as rational as quitting a job to avoid rush hour. Sure, it's possible and may even be an idea with merit, but it isn't at all practical.