In 1915, two years before the entry of the United States into World War I, he joined the American Field Service, an ambulance corps that served on the French battlefields. He was later decorated with the Croix de Guerre by the French government for bravery at Verdun. For 10 years, between 1910 and 1920, Peirce lived the expatiate life in France and Paris, before returning to the United States for a couple of years. He then returned to Europe for several more years, and only returned to the U.S. permanently with the advent of World War II.