The Roman Empire was an ancient empire centered around the Mediterranean Sea, commonly dated from accession of the Emperor Augustus in 27 BC through the abdication of the last emperor in 476 AD. It was the successor state to the Roman Republic, and constituted the final period of classical antiquity.
The 500-year-old Roman Republic, which preceded it, had been weakened through several civil wars. Several events are commonly proposed to mark the transition from Republic to Empire, including Julius Caesar's appointment as perpetual dictator (44 BC), and the Battle of Actium (2 September 31 BC), though the Roman Senate's granting to Octavian the honorific Augustus is most common (16 January 27 BC).
The first two centuries of the empire were characterized by the Pax Romana, which was a period of unprecedented peace and prosperity. Though Roman expansion was mostly accomplished under the republic, it continued under the emperors. Notably, parts of northern Europe were conquered in the 1st century AD, while Roman dominion in Europe, Africa and especially Asia was strengthened during this time. Numerous uprisings were successfully put down, notably those in Britain and Judea, though the latter uprising triggered the suicide of the unpopular Emperor Nero and a brief civil war.
The empire would reach its greatest territorial extent under the emperor Trajan in 117 AD, though most of his gains were given up under his successor. In the view of Dio Cassius, a contemporary observer, the accession of the Emperor Commodus in 180 AD marked the descent "from a kingdom of gold to one of rust and iron", a famous comment which has led some historians, notably Edward Gibbon, to take Commodus' reign as the beginning of the decline of the Roman Empire. A succession of unsuccessful emperors followed, and then a period of civil wars and social unrest during the Crisis of the Third Century.
The Crisis of the Third Century is a commonly applied name for the crumbling and near collapse of the Roman Empire between 235-284 C.E. It is also called the period of the “military anarchy”. After Augustus Caesar declared an end to the Civil Wars of the first century B.C.E., the Empire had enjoyed a period of limited external invasion, internal peace and economic prosperity.
In the third century, however, the Empire underwent military, political and economic crises and almost collapsed. There was constant barbarian invasion, civil war, and runaway hyperinflation. Part of the problem had its origins in the nature of the Augustan settlement. Augustus, intending to downplay his position, had not established rules for the succession of emperors. Already in the first and second century disputes about the succession had lead to short civil wars, but in the third century these civil wars became a constant factor, as no single candidate succeeded in quickly overcoming his opponents or holding on to the Imperial position for very long. Between 235-284 C.E. no fewer than 25 different emperors ruled Rome. All but two of these emperors were either murdered or killed in battle. The organization of the Roman military, concentrated on the borders, could provide no remedy against foreign invasions once the invaders had broken through. A decline in citizens' participation in local administration forced the Emperors to step in, gradually increasing the central government's responsibility.
In the late 3rd century, the emperor Diocletian stabilized the empire and established the practice of dividing authority between four co-emperors, known as the tetrarchy. Disorder began again soon after his reign, but order was restored by