Rome reached its greatest territorial expanse under Trajan (AD 98–117) a period of increasing trouble and decline began under Commodus (177–192). The first two centuries of the Empire saw a period of unprecedented stability and prosperity known as the Pax Romana ( lit. The vast Roman territories were organized in senatorial and imperial provinces. The Roman Senate granted Octavian overarching power ( imperium) and the new title of Augustus, marking his accession as the first Roman emperor of a Principate with Italia as the metropole and Rome as its sole capital. The Roman Republic became severely destabilized in civil wars and political conflicts, eventually culminating in the victory of Octavian over Mark Antony and Cleopatra at the Battle of Actium in 31 BC and the subsequent conquest of the Ptolemaic Kingdom in Egypt. The adoption of Christianity as the state church in 380 and the fall of the Western Roman Empire conventionally marks the end of classical antiquity and the beginning of the Middle Ages. It included territory around the Mediterranean in Europe, North Africa, and Western Asia, and was ruled by emperors.
The Roman Empire was the post- Republican state of ancient Rome.