Definition
The Roman Empire is the phase of Roman history that began when Augustus became Rome's first emperor in 27 BCE and the Republic's shared power gave way to rule by one man. Emperors held supreme authority over the army, the law, and the provinces, though they often preserved republican forms as a polite fiction.
Under the emperors, Rome reached its largest size and, for roughly two centuries, an unusual stretch of internal stability. The empire later split administratively, and its western half dissolved in 476 CE, while the eastern half continued for another thousand years.
Why it matters
How it works
Imperial government rested on the emperor's control of the legions. Loyalty of the army was the foundation of power, which made succession a recurring weakness — there was no fixed rule for who became emperor, and disputed successions repeatedly turned into civil war.
To manage its size, the empire was eventually divided into eastern and western administrations. The wealthier, more urbanized east proved more durable; the west, facing heavier military pressure and weaker finances, broke into successor kingdoms during the fifth century CE.