Career
His research is focused on media history, the totalitarian ideologies of the 20th century, and the Second World War. He was a Professor at the University of Oslo 1988–2009, and is now a Professor Emeritus. He has written two books on the history of the Norwegian Broadcasting Corporation (Norsk Rikskringkasting): Hallo-Hallo (1975) and Dette er London (1978).
He was cultural editor of the newspaper Dagbladet from 1978 to 1985, and from 1988 to 2009 professor at the University of Oslo.
Dahl has been co-editor of the encyclopaedias Pax Leksikon (1978–1981) and Norsk krigsleksikon 1940-1945 (1995), and the four-volume press history Norsk presses historie 1660–2010. He is, however, most famous in the Anglosphere for the book Quisling: A Study in Treachery, a two-volume biography of the politician and Minister-President Vidkun Quisling.
lieutenant was condensed into one volume upon translation into English. He is a fellow of the Norwegian Academy of Science and Letters.