The Roman Empire ruled the Mediterranean and much of Europe, Western Asia and North Africa. The Romans conquered most of this during the Republic, and it was ruled by emperors following Octavian's assumption of effective sole rule in 27 BC. The western empire collapsed in 476 AD, but the eastern empire lasted 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. The Gallic and Palmyrene empires broke away from the state and a series of short-lived emperors led the Empire, which was later reunified under Aurelian (r. 270–275).

Gaius Julius Caesar (12 or 13 July 100 BC – 15 March 44 BC) was a Roman general and statesman. A member of the First Triumvirate, Caesar led the Roman armies in the Gallic Wars before defeating his political rival Pompey in a civil war. He subsequently became dictator from 49 BC until his assassination in 44 BC. Caesar played a critical role in the events that led to the demise of the Roman Republic and the rise of the Roman Empire.

In 60 BC, Caesar, Crassus, and Pompey formed the First Triumvirate, an informal political alliance that dominated Roman politics for several years. Their attempts to amass political power were opposed by many in the Senate, among them Cato the Younger with the private support of Cicero. Caesar rose to become one of the most powerful politicians in the Roman Republic through a string of military victories in the Gallic Wars, completed by 51 BC, which greatly extended Roman territory. During this time, he both invaded Britain and built a bridge across the river Rhine. These achievements and the support of his veteran army threatened to eclipse the standing of Pompey. The alliance between Caesar and Pompey slowly broke down and, by 50 BC, Pompey had realigned himself with the Senate. With his command expiring and the Gallic Wars largely concluded, the Senate ordered Caesar to step down from his military command and return to Rome.

Gaius Julius Caesar Augustus (born Gaius Octavius; 23 September 63 BC – 19 August AD 14), also known as Octavian (Latin: Octavianus), was the founder of the Roman Empire, who reigned as the first Roman emperor from 27 BC until his death in AD 14. The reign of Augustus initiated an imperial cult and an era of imperial peace (the Pax Romana or Pax Augusta) in which the Roman world was largely free of armed conflict. The Principate system of government was established during his reign and lasted until the Crisis of the Third Century.

Octavian was born into an equestrian branch of the plebeian gens Octavia. Following his maternal great-uncle Julius Caesar's assassination in 44 BC, Octavian was named in Caesar's will as his adopted son and heir, and inherited Caesar's name, estate, and the loyalty of his legions. He, Mark Antony, and Marcus Lepidus formed the Second Triumvirate to defeat the assassins of Caesar. Following their victory at the Battle of Philippi (42 BC), the Triumvirate divided the Roman Republic among themselves and ruled as de facto dictators. The Triumvirate was eventually torn apart by the competing ambitions of its members; Lepidus was exiled in 36 BC, and Antony was defeated by Octavian at the Battle of Actium in 31 BC. Antony and his wife Cleopatra, the Ptolemaic queen of Egypt, killed themselves during Octavian's invasion of Egypt, which then became a Roman province.

After the demise of the Second Triumvirate, Augustus restored the outward facade of the free republic, with governmental power vested in the Roman Senate, the executive magistrates and the legislative assemblies, yet he maintained autocratic authority by having the Senate grant him lifetime tenure as commander-in-chief, tribune and censor. A similar ambiguity is seen in his chosen names, the implied rejection of monarchical titles whereby he called himself Princeps Civitatis 'First Citizen' juxtaposed with his adoption of the name Augustus.