When Germany invaded France in 1914 Ravel tried to join the French Air Force. He considered his small stature and light weight ideal for an aviator, but was rejected because of his age and a minor heart complaint. While waiting to be enlisted, Ravel composed "Trois Chansons", his only work for a cappella choir, setting his own texts in the tradition of French 16th-century chansons. He dedicated the three songs to people who might help him to enlist. After several unsuccessful attempts to enlist, Ravel finally joined the Thirteenth Artillery Regiment as a lorry driver in March 1915, when he was forty. Stravinsky expressed admiration for his friend's courage: "at his age and with his name he could have had an easier place, or done nothing". Some of Ravel's duties put him in mortal danger, driving munitions at night under heavy German bombardment. At the same time his peace of mind was undermined by his mother's failing health. His own health also deteriorated; he suffered from insomnia and digestive problems, underwent a bowel operation following amoebic dysentery in September 1916, and had frostbite in his feet the following winter.