BC by the Ubii. Cologne became acknowledged, as a city by the Romans in AD 50, by the name of Colonia Claudia Ara Agrippinensium. From the death of Augustus in AD 14 until after AD 70, Rome accepted as her Germanic frontier the water-boundary of the Rhine and upper Danube. Beyond these rivers she held only the fertile plain of Frankfurt, opposite the Roman border fortress of Moguntiacum (Mainz), the southernmost slopes of the Black Forest and a few scattered bridge-heads. The northern section of this frontier, where the Rhine is deep and broad, remained the Roman boundary until the empire fell. The southern part was different. The upper Rhine and upper Danube are easily crossed. The frontier which they form is inconveniently long, enclosing an acute-angled wedge of foreign territory between the modern Baden and Württemberg. The Germanic populations of these lands seem in Roman times to have been scanty, and Roman subjects from the modern Alsace-Lorraine had drifted across the river eastwards. The Romans kept eight legions in five bases along the Rhine. The number was reduced to four as more units were moved to the Danube. The actual number of legions present at any base or in all, depended on whether a state or threat of war existed. Between about AD 14 and 180, the assignment of legions was as follows: For the army of Germania Inferior, two legions at Vetera (Xanten), I Germanica and XX Valeria (Pannonian troops); two legions at oppidum Ubiorum ("town of the Ubii"), which was renamed to Colonia Agrippina, descending to Cologne, V Alaudae, a Celtic legion recruited from Gallia Narbonensis and XXI, possibly a Galatian legion from the other side of the empire. For the army of Germania Superior: one legion, II Augusta, at Argentoratum (Strasbourg); and one, XIII Gemina, at Vindonissa (Windisch). Vespasian had commanded II Augusta, before he became emperor. In addition, were a double legion, XIV and XVI, at Moguntiacum (Mainz). The two original military districts of Germania Inferior and Germania Superior, came to influence the surrounding tribes, who later respected the distinction in their alliances and confederations. For example, the upper Germanic peoples combined into the Alemanni. For a time, the Rhine ceased to be a border, when the Franks crossed the river and occupied Roman-dominated Celtic Gaul, as far as Paris. Germanic tribes crossed the Rhine in the Migration period, by the 5th century establishing the