Complex sentence: Convincing an American general that he is sincere, when the United States enters the war, he joins the American Air Service and finally flies a mission with the Lafayette Escadrille, where he proves to be a superb fighter pilot.
Split into simpler sentences: Convincing an American general that he is sincere, when the United States enters the war, he joins the American Air Service. Walker finally is able to fly a mission with the Lafayette Escadrille, where he proves to be a superb fighter pilot.