The Roman Empire's need to mobilize continuously and more exactingly than before its economic, financial and human resources in order to meet the demands of collective defence, entailed a strengthening of the administrative structure at all levels-central, provincial and local. The revision of traditional theories has had the effect of modifying the chronological periodization, by showing the Severi to have been more continuators of the Antonines than precursors of Diocletian, and by redefining the tetrarchy itself as a phase in the transition between the classical imperial system and the most characteristic innovations of the late empire, which do not appear before Constantine. Throughout the whole period the diplomatic relations maintained by the various emperors with the eastern cities, in the purest traditions of hellenism seem to conform to the ideal expressed by Aelius Aristides some decades earlier. Severan reforms are said to have revised the political presuppositions and the juridical configuration of the land tax.