In 1917, when the United States entered World War I, Porter moved to Paris to work with the Duryea Relief organization. Some writers have been skeptical about Porter's claim to have served in the French Foreign Legion, but the Legion lists Porter as one of its soldiers and displays his portrait at its museum in Aubagne. By some accounts, he served in North Africa and was transferred to the French Officers School at Fontainebleau, teaching gunnery to American soldiers. An obituary notice in "The New York Times" stated that, while in the Legion, "he had a specially constructed portable piano made for him so that he could carry it on his back and entertain the troops in their bivouacs." Another account, given by Porter, is that he joined the recruiting department of the American Aviation Headquarters, but, according to his biographer Stephen Citron, there is no record of his joining this or any other branch of the forces.