An attorney when the war began, Thieffry joined the army but was soon captured by the Germans. He escaped on a stolen motorcycle and was interned when he entered the Netherlands. Employing all his legal skills, he successfully argued for his release and was promptly back on the stolen motorcycle, heading for home. In July 1915, Thieffry transferred to the Belgian Air Service where he crashed more aircraft during training than any other Belgian pilot. As a result, his superiors were reluctant to assign him to a two-seater squadron for fear he would kill the observer in a crash. Instead, he was assigned to fly single-seat fighters. Thieffry soon crashed his first Nieuport scout and as he attempted to extract himself from the wreckage, he inadvertently fired his machine gun, scattering the onlookers who were rushing to his aid. His skills as a pilot eventually improved and Thieffry went on to become an ace. In February 1918, he was shot down in flames but survived and was captured.