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.
Constitutional reforms of Julius Caesar
After Caesar's assassination, Mark Antony, who at the time had been Caesar's fellow consul, eventually formed an alliance with Caesar's adopted son and great-nephew, Gaius Octavian. Along with Marcus Aemilius Lepidus, who was Caesar's magister equitum (master of horse) at the time of his assassination, they formed an alliance known as the Second Triumvirate. They held powers that were nearly identical to the powers that Caesar had held under his constitution, and as such, the senate and assemblies remained powerless. The conspirators were defeated at the Battle of Philippi in 42 BC. Lepidus became powerless, and Antony went to Egypt to seek glory in the east, while Octavian remained in Rome. Eventually, however, Antony and Octavian fought against each other in one last battle. Antony was defeated in the naval Battle of Actium in 31 BC, and committed suicide in 30 BC. In 29 BC, Octavian returned to Rome, as the unchallenged master of the state. In 27 BC, Octavian offered to give up the Dictatorial powers which he had held since 42 BC, but the senate refused, and thus ratified his status as master of the state. He became the first Roman Emperor, Augustus, and the transition from Roman Republic to Roman Empire was complete.