Establishment
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 A.D. 193, Severus took special interest in refortifying the northern border in Britannia, and in 208 he relocated in Eboracum to oversee the military campaigns to pacify 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 places the date of the split occurred in 197, though 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.
Read more about this topic: Britannia Inferior