Physical boundaries of the Empire in the period 324-527
  The distribution of the Late Roman empire among Constantine I's three sons, Constantine II (337-340), Constans (337-350) and Constantius II (337-361), in 337, initiated the growing division between its western and eastern parts, and facilitated the secession of the former. Until the mid-fifth century, however, the Mediterranean remained a Roman Sea'. The Empire streched from Great Britain and the Gibraltar strait in the west, to Mesopotamia in the east, and Nubia in the south. It was divided into four large administrative units, the prefectures (praefecturae praetorio), as well as in numerous subdivisions, the dioceses (dioeceses) and the provinces (provinciae). The prefecture of the East (per Orientem) included the territories of Asia Minor, the Near East, Egypt and Libya; the prefecture of the Illyricum (per Illyricum), roughly the territory of the nothern Balkans (except southern Greece, which was directly administered by the emperor). The prefecture of Illyricum, Italy and Africa (Illyrici, Italiae et Africae) comprised the homonymous territories, with Illyricum' refering here to the western Balkans. The prefecture of the Gauls (Galliarum or per Gallias) constisted of Spain, Gaul, Germany and Great Britain.
  Theodosios I (379-395), too, bequeathed the empire to his two sons Arkadios (395-408) and Honorius (395-424). The administrative frontier between the eastern (Pars Orientis) and the western territories (Pars Occidentis) ran, in a north-south direction, through the Balkan lands, from the town of Sirmium in the province of Pannonia, along the eastern boundary of the province of Dalmatia, and, further south, on the African coast, along the common border of the provinces of Tripolitania and Upper Libya.