The Roman Empire was a state that controlled the Mediterranean and much of Europe, Western Asia, and North Africa during the classical period. The Roman Republic had previously conquered most of these territories, which became ruled by emperors following triumvir Octavian's rise to power and establishment of a Principate regime in 27 BC. By the 4th century AD the empire split into western and eastern halves. The Western Empire collapsed in 476 AD, while the Eastern Empire endured until the fall of Constantinople in 1453. By 100 BC, the city of Rome had expanded its rule from the Italian peninsula to most of the Mediterranean and beyond. However, it was severely destabilised by civil wars and political conflicts, which culminated 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. In 27 BC, the Roman Senate granted Octavian overarching military power (imperium) and the new title of Augustus, marking his accession as the first Roman emperor. The vast Roman territories were organized into senatorial provinces, governed by proconsuls who were appointed by lot annually, and imperial provinces, which belonged to the emperor but were governed by legates. The first two centuries of the Empire saw a period of unprecedented stability and prosperity known as the Pax Romana (lit. 'Roman Peace'). Rome reached its greatest territorial extent under Trajan (r. 98–117 AD), but a period of increasing trouble and decline began under Commodus (r. 180–192). In the 3rd century, the Empire underwent a 49-year crisis that threatened its existence due to civil war, plagues and barbarian invasions.