He then studied history at the Sorbonne in Paris and worked as a journalist in France and for Radio Free Europe in West Germany.
In 1956, unable to gain residence in France, he settled in New York City where he studied modern European history at Columbia University under Fritz Stern.
(1958) and then a Ph.D. (1964), with a dissertation on "Weimar Germany's 'homeless Left': The world of Carl von Ossietzky," and spent the next 33 years teaching at Columbia.
His publications include Weimar Germany's Left-wing Intellectuals (1968); The Lawful Revolution: Louis Kossuth and the Hungarians, 1848-1849 (1979); Beyond Nationalism: A Social and Political History of the Habsburg Officer Corps, 1848-1918 (1990); and Essays on Hitler's Europe (2001).
He edited and partly wrote, together with Jan T. Gross and Tony Judt, The Politics of Retribution in Europe: World War II and Its Aftermath (2000).