From the second to the ninth century.

The seven centuries which elapsed between the rule of Hadrian and that of Charlemagne saw profound changes in the geography of Europe. The Roman empire, at the height of its power and prestige in the second century, faced increasing difficulties from late in the third. It was under pressure from without. Germanic peoples were threatening its frontiers in Europe, while to the east the empire never succeeded in defining its boundary against the Parthians and other peoples of the Middle East. Within the empire the constitutional role of the emperors was not defined, and no law of succession was ever promulgated. Some emperors nominated their successors, but their choice was always liable to be challenged or overthrown by the unruly soldiery. There were times when one might say that the choice of emperor devolved upon the praetorian guard, a pampered corps of elite soldiers which was stationed just outside the walls of the city of Rome and served as an imperial guard. POLITICAL GEOGRAPHY The vast extent of the empire, from northern Britain to the Euphrates and from the Danube southward to the Sahara, would have posed profound problems even without dangers from barbarian enemies without and from weak and incompetent rulers within. In the later centuries of the empire, its resources were stretched to the limit. Its old provincial structure was with only minor changes retained, but for administrative purposes the empire was divided into two – at one time even into four – parts, with as many coequal emperors. [ABSTRACT FROM AUTHOR]