Of signal importance was the increase in the power of the praetorian prefecture at this time. This office was for a number of years held by a single prefect, Publius Fulvius Plautianus, whose daughter was married to the eldest son of Severus. However, his great power proved his undoing, and in 205 A. D. he was executed on a charge of treason made by his own son-in-law. At his death two prefects were again appointed, one of whom was Papinian, the greatest of all Roman jurists. His appointment seems to indicate a division between the military and the civil functions of the prefecture. For from this time the prefect exercised supreme jurisdiction over criminal cases in Italy beyond the hundredth milestone from the city, and in the matter of appeals from the judgments of provincial governors. In the absence of the princeps he also presided over the imperial judicial council. Following Papinian other eminent jurists filled this office. Furthermore, the supervision of the transportation of grain to Rome was transferred from the prefect of the grain supply to the praetorian prefect, and the former officer merely supervised its distribution within the city.
*War in Britain, 208-211 A. D.* Like Hadrian, Severus paid great attention to strengthening the frontier defences of the empire, particularly the fortifications which linked the Rhine and the Danube. In 208 A. D. when Britain was invaded by the Caledonians, he took the field, accompanied by his two sons. He reinforced Hadrian's earthen wall between the Tyne and the Solway by a wall of stone, and carried on guerilla warfare against the tribes of the northern part of the island. However, they had not been completely pacified when he died at York in February, 211 A. D., leaving the principate to his sons, Caracalla and Geta, both of whom had previously received the title of Augustus.
*Caracalla, 211-217 A. D.* The bitter enmity which had long existed between the two brothers continued during a year of joint rule, and divided the empire into rival factions. Then Caracalla, who had previously sought to make himself sole ruler, succeeded in having Geta assassinated. Many of the latter's friends, among them the prefect Papinian, were executed. Caracalla was cruel and vicious, and displayed no capacity for governing. He relied solely upon the goodwill of the soldiery and courted their support by increased pay and lavish donatives. In 212 A. D., by the famous Antonian Constitution (_constitutio Antoniniana_) he extended Roman citizenship to all the provincials of the empire, except those who were in a condition of vassalage, such as some of the barbarian peoples who had been settled on waste lands within the Roman borders, and not citizens of organized municipalities (_dediticii_). This act was the logical culmination of the policy of his predecessors who had granted citizenship to many provincial municipalities and had sanctioned its automatic extension to soldiers of the legions and auxiliary corps. Perhaps Caracalla's chief motive was to supply a fresh source of income for the treasury, which was sadly depleted by his extravagances, for he greatly increased the number of those liable to the five per cent inheritance tax which fell only upon Roman citizens. A second motive may well have been the desire to secure a uniformity of legal status and of municipal organization throughout the empire.