Angrivarii
The wars began in the last years of the reign of Augustus, first emperor of Rome. Augustus died an old but respected man in the year 14 and was celebrated with much pomp and splendor. He left a document to be read to the senate posthumously, expressly forbidding extension of the empire beyond the Rhine. News of the will was welcomed by the Germans, thinking it gave them a free hand in the region. Germanicus found it necessary to pacify the border, which he did by a combination of scorched earth raids and offers of alliance with Rome - in short, stick and carrot. These raids also kept the army of the lower Rhine distracted from the possibility of mutiny, which had broken out on Augustus's death and only been quelled by concessions and executions.
Augustus
A final reason for the Second Settlement was to give the Principate constitutional stability and staying power in case something happened to Princeps Augustus. His illness of early 23 BC and the Caepio conspiracy showed that the regime's existence hung by the thin thread of the life of one man, Augustus himself, who suffered from several severe and dangerous illnesses throughout his life. If he were to die from natural causes or fall victim to assassination, Rome could be subjected to another round of civil war. The memories of Pharsalus, the Ides of March, the proscriptions, Philippi, and Actium, barely twenty-five years distant, were still vivid in the minds of many citizens. Proconsular imperium was conferred upon Agrippa for five years, similar to Augustus' power, in order to accomplish this constitutional stability. The exact nature of the grant is uncertain but it probably covered Augustus' imperial provinces, east and west, perhaps lacking authority over the provinces of the Senate. That came later, as did the jealously guarded tribunicia potestas. Augustus' accumulation of powers were now complete. In fact, he dated his 'reign' from the completion of the Second Settlement, 1 July 23 BC.