Born into a German Jewish middle class family in Berlin in 1889, he excelled at the Gymnasium before studying at the Friedrich-Wilhelms-Universität in Berlin with Otto Hirschfeld and Eduard Meyer. Soon, he established himself as an expert in Roman constitutional history. In 1914, Rosenberg proved to be a conformist representative of the German academy, believing in the "ideas of 1914," and signing nationalist petitions. He then was drafted into the army, working for the Kriegspresseamt (the army's public relations office).
After Germany's defeat in 1918, he joined the new Independent Social Democratic Party (USPD) and then on its formation in 1920 the KPD, the German Communist Party. He was on the Executive Committee of the Third International and for a period a member of the Central Committee of the German Communist Party. He was strongly influenced by Karl Korsch and later like Korsch held that Stalinist Russia had become 'state capitalist' as a result of the First Five Year Plan. In 1927 he was expelled by the German Communist Party and withdrew from revolutionary politics, becoming a democratic socialist. In 1931 he was finally made a Professor of History at Berlin University. When the Nazis came to power in 1933 he left for Switzerland, and then spent three years in exile in Britain from 1934-37 teaching at the University of Liverpool, before moving to America, dying in New York in 1943 as Professor of Brooklyn College. In the 1960s, his books came to more widespread attention among German historians.
