query: Who was the first emperor of the Roman Empire?
passage: List of Roman emperors
The Roman emperors were rulers of the Roman Empire, wielding power over its citizens and military, dating from the granting of the title of Augustus to Gaius Julius Caesar Octavianus by the Roman Senate in 27 BC. Augustus maintained a façade of republican rule, rejecting monarchical titles but calling himself princeps senatus (first man of the senate)[1] and princeps civitatis (first citizen of the state). The title of Augustus was conferred on his successors to the imperial position. The style of government instituted by Augustus is called the Principate and continued until reforms by Diocletian. The modern word 'emperor' derives from the title imperator, which was granted by an army to a successful general; during the initial phase of the empire, it still had to be earned by the princeps.
passage: Roman emperor
The legitimacy of an emperor's rule depended on his control of the army and recognition by the Senate; an emperor would normally be proclaimed by his troops, or invested with imperial titles by the Senate, or both. The first emperors reigned alone; later emperors would sometimes rule with co-emperors and divide administration of the empire between them.