In Asia the Roman frontiers were crumpling back under the push of a renascent Persia. This new Persia, the Persia of the Sassenid kings, was to be a vigorous and on the whole a successful rival of the Roman Empire in Asia for the next three centuries.
A glance at the map of Europe will show the reader the peculiar weakness of the empire. The river Danube comes down to within a couple of hundred miles of the Adriatic Sea in the region of what is now Bosnia and Serbia. It makes a square re-entrant angle there. The Romans never kept their sea communications in good order, and this two hundred mile strip of land was their line of communication between the western Latin-speaking part of the empire and the eastern Greek-speaking portion. Against this square angle of the Danube the barbarian pressure was greatest. When they broke through there it was inevitable that the empire should fall into two parts.
A more vigorous empire might have thrust forward and reconquered Dacia, but the Roman Empire lacked any such vigour. Constantine the Great was certainly a monarch of great devotion and intelligence. He beat back a raid of the Goths from just these vital Balkan regions, but he had no force to carry the frontier across the Danube. He was too pre-occupied