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.
Emperor
When Republican Rome turned into a "de facto" monarchy in the second half of the 1st century BC, at first there was no name for the title of the new type of monarch. Ancient Romans abhorred the name Rex ("king"), and it was critical to the political order to maintain the forms and pretenses of republican rule. Julius Caesar had been Dictator, an acknowledged and traditional office in Republican Rome. Caesar was not the first to hold it, but following his assassination the term was abhorred in Rome.
Augustus, considered the first Roman emperor, established his hegemony by collecting on himself offices, titles, and honours of Republican Rome that had traditionally been distributed to different people, concentrating what had been distributed power in one man. One of these offices was "princeps senatus", ("first man of the Senate") and became changed into Augustus' chief honorific, "princeps civitatis" ("first citizen") from which the modern English word and title prince is descended. The first period of the Roman Empire, from 27 BC – 284 AD, is called the "principate" for this reason. However, it was the informal descriptive of "Imperator" ("commander") that became the title increasingly favored by his successors. Previously bestowed on high officials and military commanders who had "imperium", Augustus reserved it exclusively to himself as the ultimate holder of all "imperium". ("Imperium" is Latin for the authority to command, one of a various types of authority delineated in Roman political thought.)