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.
Reign of Marcus Aurelius
Although Marcus Aurelius shows no personal affection for Hadrian (significantly, he does not thank him in the first book of his "Meditations"), he presumably believed it his duty to enact the man's succession plans. Thus, although the senate planned to confirm Marcus Aurelius alone, he refused to take office unless Lucius received equal powers. The senate accepted, granting Lucius the "imperium", the tribunician power, and the name Augustus. Marcus Aurelius became, in official titulature, Imperator Caesar Marcus Aurelius Antoninus Augustus; Lucius, forgoing his name Commodus and taking Marcus Aurelius' family name, Verus, became Imperator Caesar Lucius Aurelius Verus Augustus. It was the first time that Rome was ruled by two emperors.