of Augustus’s policy in not enforcing their universal applicability at an earlier date. The Thracians, for example, rose in open revolt when they were first summoned to supply a contingent for service at a distance from their own borders.[27] Somewhat similar was the fate of the border militia on the Rhine and Danube. On the latter frontier the revolt of 6-9 showed at an early date the dangers of the system. After its suppression the clan chiefs seem, in many cases at any rate, to have been deposed and replaced by Roman officials,[28] regiments of Pannonians and Dalmatians were raised and transferred to other provinces, and a garrison was imported from outside to control the country.[29] On the Rhine the process was a more gradual one. The Batavi, for example, whom we have noticed serving in the campaigns of Germanicus as a clan levy under their _dux_ Chariovalda, seem to have been organized in regular auxiliary regiments by the middle of the first century,[30] although they still retained, in common with many other corps of Rhenish auxilia, their clan chiefs as their _praefecti_.[31] In the year 69 we also find the Helvetii still responsible for maintaining the garrison of a fort within their borders, and a militia existing in Raetia capable of supplementing the garrison of regular auxilia.[32] Some even of the Gallic states, which were more distant from the frontier, sent contingents to support Vitellius, which were not, however, regarded as a very sensible addition to his forces.[33] Probably these last vestiges of independent organization and control were swept away at the time of the reorganization of the Rhine army in 70, after the rebellion of Civilis. From this date onward there are at any rate few traces here or elsewhere of any use of irregulars of this type by the military authorities. This militia, which might have proved invaluable in the days to come as a national reserve, fell a victim, together with the local autonomy on which it was based, to the growing tendency towards centralization which marks the first and second centuries. The supersession of local officials by the agents of a centralized bureaucracy in the civil administration coincides with the complete transference of the burden of the defence of the state to the shoulders of a professional army. It is the purpose of the following chapters to discuss one part of this army, the auxilia, to trace its organization and the part which it played in frontier defence, and to illustrate from this study the lines on which the military system of the Empire developed and the causes of its failure. Footnotes: [1] Livy, xxii. 37 ‘Milite atque equite scire nisi Romano Latinique nominis non uti populum Romanum: levium armorum auxilia etiam externa vidisse in castris Romanis’. Cf. Polybius, iii. 75.