Far in the East round the mouths of the Danube the predecessors of the Cossacks on their little horses kept the Roman outposts in a state of terror. Ovid tells us how they swooped down upon the labourers in the fields round the camp at Tomi, how their arrows fell into its very centre, how they galloped round its walls, picked up some unfortunate straggler, and were off with him before pursuit could be organized. Reading such a description as this we realize the true significance of the two Roman walls in England, and the wall from the Main to the Danube in Germany. They were not defences against systematic war; they were too long to be defended against an organized invasion, but they effectually prevented raiding. Cattle cannot be lifted over a wall twelve feet high. The difference between our frontier wars and the Roman frontier wars lies in the proximity of the Roman frontiers to the heart of the Empire; but in spite of the perpetual imminence of the danger, the Romans did not pay a sufficient tribute of gratitude to the generals who secured their safety, and were inclined to underestimate their services. Even such a clear-sighted historian as Merivale, in speaking of the military operations of Tiberius and Drusus in Germany, adopts the attitude of Tacitus, and disparages the cautious policy of Augustus, which discouraged schemes of boundless conquest in Central Europe. Tacitus wrote, when Trajan was engaged in rectifying the frontier of the Lower Danube, new dangers threatened the Empire and new measures seemed advisable. The men of his day might be pardoned for thinking that they were called upon to do what Augustus had unwisely left undone. Possibly they were right, but they omitted from their calculations a fact which was of the first importance, and of itself imposed prudence. The fighting strength of the Empire was not adequate for a policy of indefinite expansion at the end of the reign of Augustus, nor even in its middle period. It was difficult to steer between the two extremes. Augustus had seen the evils of a rampant military policy in the careers of his uncle and Antonius; he had known what it was to be the puppet of his own soldiers; he had fought in the Civil Wars, and he rightly inferred that there could be no settled government so long as the sword outbalanced the gown. Quite apart from any personal ambition or mean motive, he shrank from creating fresh military heroes, who might be tempted to overthrow the carefully balanced fabric of the State, and renew the Marian and Sullan episodes, or the hateful reign of the Triumvirate in which he had himself taken an unwilling part. On the other hand, a certain strength was necessary to police the Empire and guard its frontiers. In the encouragement which he gave to civilians in the public service, in the revival of commerce, and the abundance of employment secured by the internal peace of the Empire, Augustus cut off his supply of recruits; the army no