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.
European balance of power
Rome, a Latin republic with a mixed constitution, unified Italy around the same period and then rose to prominence in the western and eastern Mediterranean through the Punic and Macedonian wars. Such a rapid expansion was followed by the so-called 'Roman Revolution', when the Gracchi, the Servile Wars, the Social War and Sulla's civil wars shook the Italian peninsula. Meanwhile, the popularity and wealth of Roman generals increased:  notably Julius Caesar acquired fame for projecting military power north of the Alps into Gaul, east of the Rhine into Germania and across the English channel into Britain. A group of senators afraid of Caesar's title of "dictator for life" assassinated him the Ides of March of 44 BC. The adoptive son of Caesar, Octavian Augustus, defeated the killers of his father and became the first Roman Emperor ("Princeps") in 27 BC.