It was not enough to try to restore better social and political conditions in Rome and Italy, not enough even to extend an adequate administrative system to the provinces: the heart and body of the empire must be shielded from attack from beyond its frontiers. This involved two problems: the establishment and maintenance of suitable frontiers and the ability to man them. The last requirement involved yet another problem: an adequate army must be created on a professional basis, and its loyalty to the central government must be guaranteed. One reason for the collapse of the Republic had been the challenge that had arisen from successful provincial governors, backed by their armies: the Principate must not be allowed to go the same way to destruction. Thus Augustus was faced with an awkward problem. The safety of the empire demanded that he should push the frontiers well back, but distant frontiers involved entrusting armies to men far from the eye of the Princeps, and long frontiers required more men and expense. Augustus therefore tried to establish as economical a frontier system as was consistent with safety, and at the same time to create an army that would guard it adequately while remaining loyal to himself. We must therefore look ﬁrst at his army reforms and then at the various provinces and the problems that their defence involved.