The Roman Empire
The imperial system of the Roman Empire depended heavily on the personality and standing of the emperor himself. The reigns of weak or unpopular emperors often ended in bloodshed at Rome and chaos throughout the empire as a whole. In the third century AD the very existence of the empire was threatened by a combination of economic crisis, weak and short-lived emperors and usurpers (and the violent civil wars between their rival supporting armies), and massive barbarian penetration into Roman territory.
Relative stability was re-established in the fourth century AD, through the emperor Diocletian's division of the empire. The empire was divided into eastern and western halves and then into more easily administered units. Although some later emperors such as Constantine ruled the whole empire, the division between east and west became more marked as time passed. Financial pressures, urban decline, underpaid troops and consequently overstretched frontiers - all of these finally caused the collapse of the western empire under waves of barbarian incursions in the early fifth century AD. The last western emperor, Romulus Augustus, was deposed in AD 476, though the empire in the east, centred on Byzantium (Constantinople), continued until the fifteenth century.