After their plane is shot down in the First World War, Captain de Boeldieu (played by Pierre Fresnay) and Lieutenant Maréchal (Jean Gabin) are sent to a concentration camp in Germany. Both come from different backgrounds - Boeldieu an aristocrat and Maréchal a working class man.
Their repeated escape attempts with fellow prisoner Rosenthal (Marcel Dalio) mean they are sent on to a seemingly impenetrable camp run by aristocrat von Rauffenstein (Erich von Stroheim).
There de Boeldieu strikes up a friendship with von Rauffenstein because of their shared backgrounds until a further escape attempt has tragic consequences for the pair.
La Grande Illusion is more about the social status of those involved in the escapes. We see men who are willing to fight for those who, outside of war, would be totally isolated from them, and we see two men pitted against each other when they may well have mingled in social circles had they not been enemies.
The name La Grande Illusion is more about the facade that war draws down over those caught up in it. People are forced to fight against those they have more in common with and battle alongside others for a common cause that neither truly believes in.
Director Jean Renoir probably hoped his 1937 film would aid in preventing the impending Second World War. It didn't, but that's not to say that it's not relevant in today's world as a thought-provoking item of propaganda for peace.