The conquest of Gaul had brought the Romans to the Rhine: but since this did not mark any clear limits of division between the peoples, it was inevitable that sooner or later the work that Caesar had believed he could stop, at least for the moment, on the banks of it, was taken up and carried further to the east. Peoples of the Germanic race equally inhabited both on this side and on the other side of the Rhine: and the exchanges between the two banks were intense and varied, and not limited only to trade: for the tribes of the right bank pushed by those located further away, and not yet fixed in their seats, they were of necessity drawn to cross the left bank and seek new lands in Gaul; on the other hand, Caesar himself, in order to have a more peaceful possession of it, had twice had to transport his legions across the river.
In 38 a. C. the Ubî, already under the protection of the Romans for some time, are forced to move to the left bank, where Agrippa assigns them lands, founding the first nucleus of what will later be the city of Cologne; for his part Agrippa crossed the river twice, in 38 and 25, and imposed his will on the tumultuous tribes hostile to the Romans. In 16 a general insurrection broke out among the peoples of the Middle Rhine (Sicambri, Usipeti and Tenteri), who, having risen up in arms, crossed the river, coming to attack the Romans in Gaul: faced by a legion commanded by the legate Marco Lollio, they defeated it., conquering the brand. Perhaps this was the decisive fact that led Augustus, who then rushed to Gaul, to directly address the problem of the pacification of the lands beyond the Rhine and the conquest as far as Elbe, not so much to add new lands to the empire, but to give the necessary security to the border of Gaul, and indirectly also to that of the regions of the upper and middle Danube, with which the first was connected. The enterprise, whose decision was certainly not extraneous to the will of Drusus, who in the 13th had assumed the government of Gaul and the command of the armies of the Rhine, began under good auspices, although through difficult difficulties. In the 12th it was the Germans themselves who seemed to take the offensive by rising again against the Romans, killing those of them who were in their territory and trying to cross the river: Drusus prevented him and passed in his turn on the right of the Rhine, subduing the Sicambri and their neighbors. But the great action planned was to take place above all through the lands of the lower Rhine and along the coast of the North Sea: a navigable channel (fossa Drusiana) had been dug to connect the Rhine delta with Lake Flevo (Zuider Zee). Already from this year Drusus therefore provided for the Batavi and the Frisî to enter under the subjection of the empire: then sailing towards the east he reached the territory of the Cauci, at the mouth of the Visurgis (Weser), which became the Roman border on the sea.
According to cancermatters, the following year, while the fleet was again operating along the coasts, the army crossed the Rhine in the middle course of it, subjugated the Sicambri, the Catti and the Cherusci, advancing to the banks of the Weser and placing a garrison in the conquered territory, to Aliso (Haltern): a surprise by the enemies during the retreat would have been fatal, if the firmness of the legionaries and the indiscipline of the Germans had not changed the probable defeat of the former into a great victory. An insurrection of the Catti in 10 gave Drusus reason to cross the Rhine the following year (9 BC): this time he went even further east: because beyond the Weser he reached the banks of the Elbe: but he died of a painful accident on the way back.
His brother Tiberius took up the inheritance, who in the following two years (8 and 7 BC) again brought the army back across the Rhine, some populations, such as the Sicambri, transplanting on this side, others rejecting farther, the whole region between the Rhine and the Elbe, pacifying and considering by now as an addition to the empire. Not that the defense line was certainly transported further eastwards, which instead remained for the most part supported by the camps located on the left of the Rhine: but beyond this a series of fortified posts was set up along the two banks of the Luppia. (Lippe), which constituted, in front of Vetera, the easiest transit route towards the interior of Germany, and above all that civil penetration began which should have made the country a completely subject region in a short time. Meanwhile, Elba also stretched from the south: having removed Tiberius from Gaul, L. Domizio Enobarbo reached the upper course of it on the Vindelicia side. Domizio Enobarbus himself was in the i d. C. governor on the Rhine, where, however, he does not seem to have accomplished great deeds. Germany was, moreover, far from being calm: the rebellion broke out among the peoples who were still far from submissive, and broke out from time to time in small local episodes.