The destructive power of war
One of the most important themes in the film is the destructive power of war. Throughout the film, viewers see how destructive war has been on countries, their landscapes, their people, and their economies (among many other things). In one way or another, the film shows viewers, war has touched and changed the lives of everyone in the world. And aside from a few minor instances (like Maréchal falling in love with a German woman named Elsa), the war has brought negativity to everyone's life.
This theme was included to warn readers against going to war. It was also included to illuminate how futile and destructive war is. At the end of World War I, little was accomplished and many lives were either permanently altered or destroyed.
The conflict between rich and poor
The Grand Illusion shows how World War I ended the aristocracy across Europe. The Grand Illusion examines the conflict between the rich and poor, which is one of the film's most significant themes. In the film, Captain de Boeldieu is a rich man and part of the aristocracy. On the other hand, Captain de Boleldieu's subordinate, and partner Lieutenant Maréchal is a poor man. Even though the two get along well with each other, they have inherent conflict in their relationship because of their class status.
Brotherhood
In popular culture and in real life, members of the military often call each other "brothers." Each of the soldiers portrayed in the film—both German and French and no matter their class—treat each other as brothers. That is: they treat each other with tremendous respect and trust, sacrifice for each other, and work hard together to accomplish a goal. They treat each other as family, essentially.
