Member of the SR Party since his schooldays in Tifiis. After graduation, lecturer in history there, 1914. Under the Provisional Government, sent to the Russian Embassy in the USA, 1917.
Remained there after the October Revolution, 1917, becoming a political refugee, and later an American citizen. Lecturer in Russian history at Harvard, from 1927, professor until 1957. Head of the Slavonic department at Harvard, 1949-1954.
Editor of one of the best Russian emigre magazines after World War II, Novyi Zhurnal, New York, 1943-1959.