After graduating in literature in Florence in 1896, he taught History at the universities of Messina (during the 1908 Messina earthquake he was the only survivor of his entire family), Pisa and Florence. From 1919 to 1921 he served in Italian Parliament. As member of the Italian Socialist Party he fought for Universal Suffrage and for the moral and economic rebirth of Italy's Mezzogiorno (southern Italy), and against corruption in politics.
After his arrest in 1925 for opposing the newly-formed Italian fascist regime, he left Italy but continued to actively organise resistance to Mussolini in France, England and finally in the USA. From 1930 to 1948 he lectured in History at Harvard University and 1940 he obtained US citizenship. Notable writings of the American years are The Fascist Dictatorship in Italy (1928), Under the Axe of Fascism (1936) and Prelude to World War II. In 1954 he retired to Italy.
He wrote mainly about recent and contemporary history, but was also noted for his studies of the medieval Italian commune. His The French Revolution: 1788-1792 (Norton, 1962) is an outstanding explanation of the social, political and philosophical currents (and monarchical incompetence) that led to that cataclysm.
He died in Sorrento in 1957.