The distinction between Urban and Prætorian cohorts, coupled with the statement of Suetonius that Tiberius placed garrisons throughout Italy, while there is no mention in Tacitus of any legion told off to the Italian service, suggests that the camp of the Prætorians at Rome only accommodated those cohorts which were on duty at the capital. It was the headquarters of the whole force, but was not habitually occupied by the whole force. It seems to have been felt that even the Prætorians were not strong enough by themselves to defend Italy in case of emergency, for there was a further provision in the shape of an arrangement with Cotys, the King of Thrace, by which he was bound to keep a force ready, if called upon, to defend northern Italy at the dangerous corner of the Adriatic. Sejanus undoubtedly showed capacity as organizing commander-in-chief in Italy, and Tiberius felt deeply the need for this assistance. He knew that the defence of the Empire was inadequate; he knew that the revenue appropriated to that defence was also inadequate, and it was for this reason that he habitually prided himself upon solving difficulties with the frontier princes by diplomacy rather than by an appeal to arms. He was thus prepared to be grateful to a man who could find a means of increasing the efficiency of the home forces without adding to their numbers. Tiberius had, in fact, serious misgivings as to the quality of the troops. Addressing the Senate early in A.D. 23, he told them that the supply of voluntary soldiers was short, and that where the numbers were adequate the morale of the men was unsatisfactory, because the recruits were generally impoverished and homeless men. Apparently, compulsory service, except in the case of special agreements with recently conquered territories, such as the Thracian kings, had been allowed to fall into abeyance, and Tiberius talked of visiting the Provinces in order to revive the compulsory levies. It is not uninteresting to note that the organizers of the Roman Empire had to meet some of our own difficulties. Men would not enlist who had anything better to do; they had, as we have seen, the further artificial difficulty that they could not draw soldiers from the working classes, who were slaves. Tacitus and his authorities, keeping their eyes fixed as usual upon Rome, do not tell us what arrangements were made for the rest of Italy, but it is not probable that the use of the barrack system was confined to the capital; the same cause will have everywhere been followed by the same results, and have demanded the same remedy. The innovation was an important one, for though the legions on active service, or in disturbed districts or imperfectly subjugated countries, lived in permanent camps, and though the military colonies in Italy had had something of the same character, a permanent standing army with permanent barracks was a new thing.