In 1953, his last major historic study, "Prelude to World War II", was published about the Second Italo-Ethiopian War. As a historian, he wrote mainly about recent and contemporary history, but he was also noted for his studies of the medieval Italian commune. His "The French Revolution: 1788–1792" is an outstanding explanation of the social, political and philosophical currents (and monarchical incompetence) that led to that cataclysm.