Located in modern northern England, the region was governed from the city of Eboracum (modern York) by a praetorian legate in command of a single legion stationed in the city. This subdivision of Britannia lasted throughout the Severan dynasty until the reorganisation of the empire under Diocletian in 296.
During the reign of Commodus, the defences along the northern border of the empire in Britannia fell into neglect and disrepair. The peace of the region was further disturbed in the tumultuous period after Commodus' death as the military power vacuum on the continent distracted the defensive legions stationed in Britannia. After his accession in 193, Severus took special interest in refortifying the northern border in Britannia, and in 208 he moved to Eboracum to oversee the military campaigns against the northern tribes. While there is some confusion as to the exact date when the subdivision of Britannia was made, it seems clear that Severus's intentions were to break up the size of the military under the command of an individual governor (as he had done in Syria), preventing them from wielding too large a military force, or at least one that could destabilize the emperor's control. Herodian puts the date of the split in 197, although there is no evidence of this distinction being formalised in inscriptions until after the death of Severus in 211. Thus, it is likely that the division of military control in Britannia was formally established by Caracalla sometime between 211-20.
^ abThe Cambridge Ancient History, Volume XII, p. 8
^"124: The Provincial Reorganisation", Roman Civilization Volume II: Selected Readings, The Empire, ed. Naphtali Lewis and Meyer Reinhold, 3rd ed. New York: Columbia University Press, 1990, pp. 427-428.