'Le Fanion de la Légion' (The Flag of the Legion), is a French song created in 1936 by Marie Dubas, with lyrics from Raymond Asso and music from Marguerite Monnot, and which was later taken up by Edith Piaf and became identified with her.
Its garrison of thirty "gars" ("guys" or boys) comes under attack by a horde of "Salopards" ("Dirty Ones" or "Bastards") - evidently a derogatory term for Saharan tribespeople which Asso may have picked up while on actual service with the Legion.
Throughout, "Le beau fanion de la légion" ("The beautiful flag of the Legion") continues to fly from the mast (no mention of the French Tricolour).
Though the song is set in the Sahara, Asso was likely influenced by the Battle of Camarón (1867), during the failed French attempt to prop up the regime of the Emperor Maximilian in Mexico - a major founding myth familiar to anyone who served in the Legion.
Rather, the Legionnaires adopted as their own a different Edith Piaf song - "Non, je ne regrette rien" (I regret nothing) - whose words in themselves have nothing to do with the Legion but came to express their defiance when accused of atrocities and involvement in a failed coup d'etat during the Algerian War (see May 1958 crisis and Algiers putsch).