After the division of the Roman Empire in 395 AD, the two parts – Western and Eastern – drifted apart politically and militarily throughout the fifth century with each half relying mainly on its own military and economic resources. The division of the resources rendered the defence of the Eastern Roman Empire (Byzantium) ever more difficult and its geography too aggravated the problems. The northern frontier of the empire was secured by the Danube river which as a geographic barrier constituted the first echelon of defence. In addition, the Romans and later Byzantines built many forts with soldiers permanently stationed along the Danube. Nevertheless, the frontier had a vulnerable section in its north-western part: the provinces of Pannonia and Illyricum. The defence of this front did not have any geographic obstacles Constantinople could rely on. At the same time the empire had to face barbarian menace from the north Caucasus with its high mountain range separating the steppe realm from the sedentary world of the South Caucasus and the Middle East.