Rouget de Lisle singing La Marseillaise by Isidore Pils (1848).
In 1792, France declared war to Austria. Claude Joseph Rouget de Lisle was then an officer in the French army. He decided to write a war song on April 25th, 1792 to support and encourage the French troops and called it "chant de guerre pour l'armee du Rhin" ("War song for the Rhine Army").
This song became very popular and was also sung by the "federes" (volunteers) from Marseille during the storming of the Tuileries on August 10th, 1792. La Marseillaise was declared National Anthem of France on July 14th, 1795.