Though the Empire had seen periods with more than one Emperor ruling jointly before, the view that it was impossible for a single emperor to govern the entire Empire was established by Emperor Diocletian following the disastrous civil wars and disintegrations of the Crisis of the 3rd century. His ideas were instituted in Roman law by the introduction of the Tetrarchy in AD 286, which divided the position of Augustus (Emperor) into two; one in the East and one in the West, each with an appointed Caesar (junior Emperor and designated successor). Though the tetrarchic system would collapse in a matter of years, the East-West geographical administrative division would endure in one form or another for centuries to come. As such, the Western Roman Empire would exist intermittently in several periods between the 3rd and 5th centuries. Though some emperors, such as Constantine I and Theodosius I, would manage to rise to the position of Augustus in both halves and as such reunify the Empire, it would often divide again upon their deaths. After the death of Theodosius I in AD 395, the Empire was divided between his sons after which it would never again be unified. Eighty-five years later, in 480, following various invasions and the collapse of central control in the West, Zeno of the Eastern Empire recognized the reality of the Western Empire's reduced domain—effective central control had ceased to exist even in the Italian Peninsula after the depositions of Julius Nepos and Romulus Augustulus—and therefore abolished the Western court and proclaimed himself the sole emperor of the Roman Empire.
The rise of Odoacer and his germanic foederati to rule over Italy in 476 was popularized by the eighteenth-century historian Edward Gibbon as a demarcating event for the end of the Western Empire and is sometimes used to mark the transition from Antiquity to the Middle Ages. Odoacer's Italy, and other Barbarian kingdoms, would maintain a pretence of Roman continuity through the continued use of the old Roman administrative systems and nominal subservience to the Eastern Roman court. Direct Imperial rule would be reimposed in large parts of the West, including the prosperous regions of North Africa and the ancient Roman heartland of Italy as well as parts of Hispania, in the sixth century by the armies of the Eastern Empire under Emperor Justinian I. Political upheaval in the Eastern heartlands, combined with foreign invasions and religious issues, made efforts to retain control of these territories difficult and they were gradually lost, this time for good.
Though the Eastern Empire retained territories in the south of Italy until the eleventh century, the influence that the Empire had over Western Europe had diminished significantly with the papal coronation of the Frankish king Charlemagne as "Roman Emperor" in AD 800. His imperial line would come to evolve into the Holy Roman Empire, which presented a revival of the Imperial title in Western Europe but was in no meaningful sense an extension of Roman traditions or institutions. The Great Schism of 1054 between the churches of Rome and Constantinople further diminished the authority the Emperor in Constantinople could hope to bring forth in the west.
As the Roman Republic expanded, it reached a point where the central government in Rome could not effectively rule the distant provinces. Communications and transportation were especially problematic given the vast extent of the Empire. News of invasion, revolt, natural disasters, or epidemic outbreak was carried by ship or mounted postal service, often requiring much time to reach Rome and for Rome's orders to be realized in the province of origin. For this reason, provincial governors had de facto rule in the name of the Roman Republic. Governors had several duties, including the command of armies, handling the taxes of the province and serving as a local chief judges.
Prior to the establishment of the Empire, the territories of the Roman Republic had been divided in 43 BC among the members of the Second Triumvirate: Mark Antony, Octavian and Marcus Aemilius Lepidus. Antony received the provinces in the East: Achaea, Macedonia and Epirus (roughly modern Greece, Albania and the coast of Croatia), Bithynia, Pontus and Asia (roughly modern Turkey), Syria, Cyprus, and Cyrenaica. These lands had previously been conquered by Alexander the Great; thus, much of the aristocracy was of Greek origin. The whole region, especially the major cities, had been largely assimilated into Greek culture, Greek often serving as the lingua franca.