In World War I, the Legion fought in many critical battles of the war, on the Western Front including Artois, Champagne, Somme, Aisne, Verdun (in 1917) and also suffered heavy casualties during 1918. The Legion was also in the Dardanelles and Macedonian front, and the Legion was highly decorated for its efforts. Many young foreigners, including Americans like Fred Zinn, volunteered for the Legion when the war broke out in 1914. There were marked differences between such idealistic volunteers as the poet Alan Seeger and the hardened mercenaries of the old Legion, making assimilation difficult. Nevertheless, the old and the new men of the Legion fought and died in vicious battles on the Western front, including Belloy-en-Santerre during the Battle of the Somme, where Seeger, after being mortally wounded by machine gun fire, cheered on the rest of his advancing battalion.
As most European countries and the US were drawn into the War, many of the newer "duration only" volunteers who managed to survive the first years of the war were generally released from the Legion to join their respective national armies. Citizens of the Central Powers serving with the Legion on the outbreak of war were normally posted to garrisons in North Africa to avoid problems of divided loyalties.