"The piece was written and put to melody in London in 1943 after Anna Marly heard a Russian song that provided her with inspiration. Joseph Kessel and Maurice Druon wrote the French lyrics. It was performed by Anna Marly, broadcast by the BBC and adopted by the maquis. The lyrics of the song revolve around the idea of a life-or-death struggle for national liberation, and they also carry elements of a communist political message (for example, calling upon the workers and peasants to rise up).
After the war the Chant des Partisans was so popular, it was proposed as a new national anthem for France. It became for a short while the unofficial national anthem, next to the official La Marseillaise"
Hat tip Captain Swing