During World War I he served as an officer in the US Army, where he was an interrogator of German prisoners of war.[2] In 1919 he joined the US delegation to the Paris Peace Conference, and in 1924 the League of Nations appointed him Financial Advisor to the government of Hungary.[2] With Hayford Peirce (the older brother of the painter Waldo Peirce) he published in French a pioneering study of Byzantine art, and he died just after completing his posthumously published biography, The Emperor Charles the Fifth.[3]