Roman Empire

With his enemies defeated, Octavian took the name Augustus and assumed almost absolute power, retaining only a pretense of the Republican form of government. His designated successor, Tiberius, took power without serious opposition, establishing the Julio-Claudian dynasty, which lasted until the death of Nero in 68. The territorial expansion of what was now the Roman Empire continued, and the state remained secure, despite a series of emperors widely viewed as depraved and corrupt (for example, Caligula is argued by some to have been insane and Nero had a reputation for cruelty and being more interested in his private concerns than the affairs of the state.

Their rule was followed by the Flavian dynasty. During the reign of the "Five Good Emperors" (96–180), the Empire reached its territorial, economic, and cultural zenith. The state was secure from both internal and external threats, and the Empire prospered during the Pax Romana ("Roman Peace"). With the conquest of Dacia during the reign of Trajan, the Empire reached the peak of its territorial expansion; Rome's dominion now spanned 2.5 million square miles (6.5 million km²). The Antonine Plague that swept through the Empire in 165–180 AD killed an estimated five million people.
The Roman Empire at its greatest extent under Trajan in AD 117.

The period between 193 and 235 was dominated by the Severan dynasty, and saw several incompetent rulers, such as Elagabalus.[50] This and the increasing influence of the army on imperial succession led to a long period of imperial collapse and external invasions known as the Crisis of the Third Century. The crisis was ended by the more competent rule of Diocletian, who in 293 divided the Empire into an eastern and western half ruled by a tetrarchy of two co-emperors and their two junior colleagues.

The various co-rulers of the Empire competed and fought for supremacy for more than half a century. On May 11, 330, Emperor Constantine I firmly established Byzantium as the capital of the Roman Empire and renamed it Constantinople. The Empire was permanently divided into the Eastern Roman Empire (later known as the Byzantine Empire) and the Western Roman Empire in 395.

The Western Empire was constantly harassed by barbarian invasions, and the gradual decline of the western Empire continued over the centuries. In the 4th century, the westward migration of the Huns caused the Visigoths to seek refuge within the borders of the Roman Empire. In 410, the Visigoths, under the leadership of Alaric I, sacked Rome.

The Vandals invaded Roman provinces in Gaul, Hispania, and northern Africa, and in 455 sacked Rome. On September 4, 476, the Germanic chief Odoacer forced the last Roman emperor in the west, Romulus Augustus, to abdicate. Having lasted for about 1200 years, the rule of Rome in the West came to an end.

The Eastern Empire suffered a similar fate, though not as drastic. Justinian managed to briefly reconquer Northern Africa and Italy, but Byzantine possessions in the West were reduced to southern Italy and Sicily within a few years after Justinian's death. In the east, partially resulting from the destructive Plague of Justinian, the Byzantines were threatened by the rise of Islam, whose followers rapidly conquered the territories of Syria, Armenia and Egypt during the Byzantine-Arab Wars, and soon presented a direct threat to Constantinople. In the following century, the Arabs also captured southern Italy and Sicily.

The Byzantines, however, managed to stop further Islamic expansion into their lands during the 8th century and, beginning in the 9th century, reclaimed parts of the conquered lands. In 1000 AD, the Eastern Empire was at its height: Basileios II reconquered Bulgaria and Armenia, culture and trade flourished. However, soon after the expansion was abruptly stopped in 1071 at the Battle of Manzikert. This finally led the empire into a dramatic decline. Several centuries of internal strife and Turkic invasions ultimately paved the way for Emperor Alexius I Comnenus to send a call for help to the West in 1095.

The West responded with the Crusades, eventually resulting in the Sack of Constantinople by participants in the Fourth Crusade. The conquest of Constantinople in 1204 fragmented what little remained of the empire into successor states, the ultimate victor being that of Nicaea. After the recapture of Constantinople by imperial forces, the empire was little more than a Greek state confined to the Aegean coast. The Eastern Empire came to an end when Mehmed II conquered Constantinople on May 29, 1453.