In no quarter of the empire was Augustus' warning against further expansion more carefully heeded than on the Rhine, until with the death of Nero the ‘progeny of the Cæsars’ ended, and nowhere else was a scheme of frontier defence based on this policy more rapidly and completely developed. The territory lying along the left bank of the river was treated as a military district and placed under the control of the legates who commanded the two army corps of Upper and Lower Germany. Within this area there were, until the foundation of Cologne in 50 A.D., besides the legionary camps and the smaller forts garrisoned by auxiliaries, only the native settlements of the subject German tribes, themselves liable to be called upon to assist the Imperial forces in repelling or punishing raids by their kinsmen beyond the river. The two armies were of imposing strength, consisting of four legions each and of at least an equal number of auxiliaries. The head-quarters of the lower army were at Xanten (Vetera), facing the valley of the Lippe, and the route that led up it into the heart of Germany. Here two legions were stationed, the others being at Neuss (Novæsium) and at Bonn (Bonna). The upper army had its head-quarters at Mainz (Mogontiacum), commanding the lowlands of the Main, and the road to the country of the Chatti. At Mainz, as at Xanten, were two legions, while Strassburg (Argentoratum) and Windisch (Vindonissa) had one each. With the tribes on the further side, ‘among whom,’ as Seneca says, ‘our peace ceases,’ Rome only concerned herself so far as the safety of the frontier required.