After the fall of the Republic and Octavian's accession as Augustus, the new imperial military policy dictated several expansionist efforts. Of these, the policy in Germania included pushing the frontier borders from the Rhine (Rhenus) to the deep German interior, which may have been desired along the Elbe (Albis) River. Germanic incursions into Gaul, which had been a recurring problem since Caesar's conquest in the 50's BC, gave Augustus a perfect excuse to keep the Legions from idleness. During Caesar's conquest, which included the first Roman crossing of the Rhine, hostility between Romans, Celts and various Germanic tribes hampered his progress. During Octavian's role as a triumvirate responsible for the western provinces, his Legate Marcus Vipsanius Agrippa led a considerable campaign between 39 and 38 BC against the Suebi (also old enemies of Caesar.) The final straw seems to have been a Germanic Sugambri invasion into Belgica (17 - 16 BC) which resulted in the loss, and ultimate disgrace, of one Legionary standard.
In response, Augustus sent his stepson Drusus, while Tiberius was busy in Pannonia, to oversee a reorganization of the Germania provinces (Superior and Interior), which were essentially military frontiers roughly encompassing the Rhine valley. By 12 BC Drusus crossed the river and conducted several punitive campaigns. Between 12 and 9 BC, the Sugambri, Frisians, Chauci, Cherusci and Chatti were all subjugated and Roman legions established several large bases in the deep Germanic Interior. In the summer of 9 BC, Drusus reached the Elbe and after apparently calling off the campaign for the season to return west, he fell of his horse and died. While it's unclear what Drusus' orders or goals really were, whether a punitive campaign or actual lasting conquest, complete with Roman border expansion, Rome had established at least loose control of the Germanic interior.
After the death of Drusus, Tiberius, an able and competent general, took over but only for a short time. While continuing the work of his brother, in 6 AD Tiberius inexplicably decided to retire to private life on the isle of Rhodes. Tiberius' departure created its own problems with Augustus' imperial strategy, but for the time being, all seemed mostly quiet in the newly conquered lands of Germania. There is now doubt, however, despite missing evidence from ancient sources that the conquest of Germania continued in Tiberius' absence. On at least one occasion, an army commanded by Lucius Domitius Ahenobarbus is attested to having crossed the Elbe in response to a Germanic uprising. The ancient sources, in apparent attempts to gain favor from Tiberius after his ascension, seemingly discounted the work of his successors in Germania, in order to glorify Tiberius all the more. Regardless, Tiberius finally returned to finish the conquest in 4 AD. At this point, a campaign was planned to finish off the extension of Rome's borders, filling in a previously unscathed stretch of land (Bohemia) occupied by the Marcomanni and their King Marbod. By the winter of 5 and 6 AD a large scale double pronged invasion was planned, with Tiberius planning on a northerly march from the Danube and another force marching east from the Rhine. A massive revolt in Pannonia, however, would put an abrupt halt to the plans, and Tiberius was forced to march south, leaving Publius Quintilius Varus in charge instead.
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The Roman legion (from the Latin legio, meaning levy) was the basic military unit of ancient Rome.