As the Great Depression unfolded, his attention shifted from the history of ideas and nationalism, which he studied under Meinecke, to economic cycles.
[citation needed] Neither Rosenberg nor his wife Helen (a promising concert pianist) seemed likely to secure a good career in Germany, due to a variety of factors including faculty politics at Cologne, as well as the rise of Nazism and his Jewish ancestry.
His work identified in the power structures and social relations of agrarian society in Prussia the roots of the authoritarian and undemocratic character of what he, with others, took to be the Sonderweg, or special path of modern German history.
His influence on the young generation of German historians has led to the claim he was the father of modern social history (Gesellschaftsgeschichte) in post-war Germany.
He retired in 1972, and returned for personal reasons to Germany in 1977, settling in Kirchzarten near the University of Freiburg, where he had been appointed Honorary Professor the year before.