The Roman emperors were the rulers of the Roman Empire from the granting of the name and title Augustus to Octavian by the Roman Senate in 27 BC onward.[1] Augustus maintained a facade of Republican rule, rejecting monarchical titles but calling himself princeps senatus (first man of the Senate) and princeps civitatis (first citizen of the state). The title of Augustus was conferred on his successors to the imperial position, and emperors gradually grew more monarchical and authoritarian.[2]
The style of government instituted by Augustus is called the Principate and continued until the late third or early fourth century.[3] The modern word "emperor" derives from the title imperator, that was granted by an army to a successful general; during the initial phase of the empire, the title was generally used only by the princeps.[4] For example, Augustus's official name was Imperator Caesar Divi Filius Augustus.[5] The territory under command of the emperor had developed under the period of the Roman Republic as it invaded and occupied much of Europe and portions of North Africa and the Middle East. Under the republic, the Senate and People of Rome authorized provincial governors, who answered only to them, to rule regions of the empire.[6] The chief magistrates of the republic were two consuls elected each year; consuls continued to be elected in the imperial period, but their authority was subservient to that of the emperor, who also controlled and determined their election.[7] Often, the emperors themselves, or close family, were selected as consul.[8]
After the Crisis of the Third Century, Diocletian increased the authority of the emperor and adopted the title "dominus noster" (our lord). The rise of powerful barbarian tribes along the borders of the empire, the challenge they posed to the defense of far-flung borders as well as an unstable imperial succession led Diocletian to divide the administration of the Empire geographically with a co-augustus in 286. In 330, Constantine the Great, the emperor who accepted Christianity, established a second capital in Byzantium, which he renamed Constantinople. Historians consider the Dominate period of the empire to have begun with either Diocletian or Constantine, depending on the author.[9] For most of the period from 286 to 480, there was more than one recognized senior emperor, with the division usually based on geographic regions. This division was consistently in place after the death of Theodosius I in 395, which historians have dated as the division between the Western Roman Empire and the Eastern Roman Empire. However, formally the Empire remained a single polity, with separate co-emperors in the separate courts.[10]