The reasons which compelled the imperial government to prefer foreign mercenaries to native troops were based at first on principles of internal policy, and at last on absolute necessity. Augustus feared the Roman senators and knights; Constantine had not the means of paying for good Roman soldiers; and Justinian could not have found a sufficient number of suitable recruits among the citizens of his wide-extended empire. The pivot of the administration of Imperial Rome, as of Imperial Britain, was the treasury, not the Horse-guards. The taxes paid by the citizens filled that treasury: but a soldier was exempt from taxation; consequently, it became a measure of unavoidable necessity on the part of the Roman government to prevent citizens escaping their financial burdens by becoming soldiers. Had the citizens got possession of arms, Rome could not have remained a despotism. On the other hand, the system of Roman tactics rendered it necessary to procure military recruits of a degree of physical strength far above the average standard of mankind. When the population of the empire had been divided into two widely separated social classes of wealthy citizens and poor cultivators, serfs, or slaves, the supply of recruits furnished by the richest portions of the empire became very small. The danger of employing foreign barbarians, who remained isolated amidst an innumerable population, and surrounded by hundreds of walled towns, manned by their own municipal guards, was evidently less than that of entrusting legions of slaves with arms, and teaching them habits of combination and discipline. The servile wars, which inflicted a mortal wound on the Republic, would have been renewed, and would probably have soon destroyed the Empire.[12] It is customary with historians to discourse on the impolicy of the Roman emperors in employing barbarian mercenaries; but the fact is, that their finances did not admit of their purchasing the thews and sinews required for the service any where but among the barbarians. The system certainly answered admirably for the imperial government. It upheld the tyranny of the Caesars and the terror of the Roman arms for more than a thousand years; and it might have rendered Rome immortal had she not committed suicide.