In 27 BCE, Octavian was the sole Roman leader. His leadership brought the zenith of the Roman civilization, that lasted for four decades. In that year, he took the name Augustus. That event is usually taken by historians as the beginning of Roman Empire. Officially, the government was republican, but Augustus assumed absolute powers. The Senate granted Octavian a unique grade of Proconsular imperium, which gave him authority over all Proconsuls (military governors).

Under Augustus's rule, Roman literature grew steadily in the Golden Age of Latin Literature. Poets like Vergil, Horace, Ovid and Rufus developed a rich literature, and were close friends of Augustus. Along with Maecenas, he stimulated patriotic poems, as Vergil's epic Aeneid and also historiographical works, like those of Livy. The works of this literary age lasted through Roman times, and are classics. Augustus also continued the shifts on the calendar promoted by Caesar, and the month of August is named after him. Augustus' enlightened rule resulted in a 200 years long peaceful and thriving era for the Empire, known as Pax Romana.

Despite its military strength, the Empire made few efforts to expand its already vast extent; the most notable being the conquest of Britain, begun by emperor Claudius (47), and emperor Trajan's conquest of Dacia (101–102, 105–106). In the 1st and 2nd century, Roman legions were also employed in intermittent warfare with the Germanic tribes to the north and the Parthian Empire to the east. Meanwhile, armed insurrections (e.g. the Hebraic insurrection in Judea) (70) and brief civil wars (e.g. in 68 CE the year of the four emperors) demanded the legions' attention on several occasions. The seventy years of Jewish–Roman wars in the second half of the 1st century and the first half of the 2nd century were exceptional in their duration and violence. An estimated 1,356,460 Jews were killed as a result of the First Jewish Revolt; the Second Jewish Revolt (115–117) led to the death of more than 200,000 Jews; and the Third Jewish Revolt (132–136) resulted in the death of 580,000 Jewish soldiers. The Jewish people never recovered until the creation of the state of Israel in 1948.

After the death of Emperor Theodosius I (395), the Empire was divided into an Eastern and a Western Roman Empire. The Western part faced increasing economic and political crisis and frequent barbarian invasions, so the capital was moved from Mediolanum to Ravenna. In 476, the last Western Emperor Romulus Augustulus was deposed by Odoacer; for a few years Italy stayed united under the rule of Odoacer, only to be overthrown by the Ostrogoths, who in turn were overthrown by Roman emperor Justinian. Not long after the Lombards invaded the peninsula, and Italy did not reunite under a single ruler until thirteen centuries later.