It has already been noted that the frontier system adopted in the second century had obvious defects. It can easily be seen that if the strongly guarded frontier line were broken through at any point the internal provinces were exposed to the greatest danger. In themselves they possessed no means of making a stand against an invader. Their garrisons were small, cut down in fact to the minimum quantity required for police duty, and the provincial militia, which we hear of during the first century, seems no longer to have existed except in Mauretania. In fact, now that the army was recruited almost entirely in the frontier provinces, the profession of arms must have been more unfamiliar to the inhabitants of Western Europe and Asia Minor than it has ever been since, and many a citizen of the prosperous little towns of Gaul, Africa, or the Hellenized districts of the East can never have set eyes on the imperial uniform. The situation was clearly a dangerous one, and the lesson of the Marcomannian War must have made it clear that this system could only continue if the frontier troops were supported by a strong and mobile striking force, ready to move at a moment’s notice to any threatened point. In the second century the only available regiments not occupied in frontier defence or police duty consisted of the Household Troops at Rome, i.e. the ten Praetorian cohorts and the Equites Singulares. The Guards were in fact employed by Domitian, Trajan, and Marcus on the Danube frontier, but their numbers were small, their duties were not calculated to increase their military efficiency, and they were rightly looked down upon by the trained veterans of the frontiers.[414] The gravity of the situation was grasped by Septimius Severus, who took advantage of the discredit in which the Praetorians were involved by their support of Didius Julianus to disband the old cohorts, which had been recruited in Italy and the ‘civilized’ provinces of Noricum, Macedonia, and Spain, and replace them by a _corps d’élite_ selected from the legions.[415] This force, still too small to be effective, was further strengthened by an increase in the number of the Equites Singulares,[416] and the addition of one of Severus’s new legions, the Secunda Parthica, which was henceforth stationed at Alba.[417] His successors continued the same policy: under Severus Alexander we find an officer of the Household Troops bearing the title _praepositus equitum itemque peditum iuniorum Maurorum_,[418] a title which implies the existence of at least two regiments of this character, and the _Osroeni sagittarii_, who were among this emperor’s following at the time of his murder, were so numerous that they attempted to set up a rival to Maximin and were temporarily disbanded.[419]