For long, history was mainly political history, and historical narrative was confined to an account of the most important crises in political life, or to an account of wars and great generals.

Social and Economic History of the Roman Empire (1926) and A Social and Economic History of the Hellenistic World (1941) were his pioneering works that transferred the attention of historians from military or political events to global economic or social problems that had been formerly hidden behind their surface. He summed up his vast knowledge on the subject in Iranians and Greeks in South Russia (1922) and Skythien und der Bosporus (1925).