From the failed Roman Republic rose the empire that achieved political and legal universalism. After defeating his remaining enemies, Octavian established the principate, assuming autocratic power in the Hellenistic manner. However, rather than antagonize the Senate, he disguised his rule in trappings of Republicanism, allowing the Senate an advisory and small administrative role. Under the title Augustus, Octavian adapted the eastern practice of emperor worship, creating an imperial cult that helped to unify the empire. He also reformed the army and imperial administration, launched public-works campaigns in Rome and the provinces, and permitted provincials to maintain their local customs. These policies helped the empire settle into the long Pax Romana, maintained by Augustus' Julio-Claudian successors, the subsequent Flavian dynasty, and the Five Good Emperors. This period was marked by generally constructive rule, laws improving the condition of women and slaves, and the expansion of civilized urban life throughout the empire. Despite palace upheavals in Rome, wars of conquest and defense, and multiple uprisings in the provinces, the Pax Romana persisted for over a century.

During this period imperial culture flourished. Shaped by the chaos of the civil wars, Virgil's Aeneid and Livy's histories celebrate Augustus' rule and Roman virtues, and Horace's poems blend appeals to reason and emotion, extolling the delights of moderate pleasure. Born after the civil wars, Ovid wrote erotic love poems. The Republican Tacitus attacked the imperial system in his histories, and Juvenal savagely satirized what he saw as the decay of Roman virtue. Stoicism became the major philosophy of the Pax Romana, represented by the writings of Seneca, Epictetus, and Marcus Aurelius. Roman artists absorbed and extended the Hellenistic aesthetic, and architects used Roman engineering skills to adapt and amplify their Greek inheritance. The greatest cultural achievement of the Empire was its law. Building upon the earlier jus civilie, Roman jurists developed the jus gentium, the rational universalist code that bound the Empire's disparate peoples. Cutting against these civilizing forces was the popularity of brutal blood-sports, often financed by the emperors themselves.

The Pax Romana contained the seeds of decline that ultimately caused the fall of the Empire. Rebellions in Egypt, Gaul, and Judea, severe economic defects, and technological innovation all revealed the underlying internal weakness of the empire. Further, the growing popularity of spiritualized philosophies (e.g. Neo-Platonism) Near Eastern mystery religions (e.g. Mithraism) demonstrated that Imperial universalism, based on the Greek rational tradition, was failing to meet people's emotional and spiritual needs. During the third century, Roman society began to crumble as economic dislocation, war, and military and civilian misrule undermined confidence in the state. To stem this tide, Diocletian and Constantine ruled openly as absolute monarchs, maintained a regimented state that overburdened citizens with taxes and obligations, and divided the Empire administratively into Eastern and Western halves. During the fourth and fifth centuries, a set of mutually reinforcing political, social, economic, and spiritual factors caused the steady decline of the Empire. These factors caused a gradual shift from a centralized, urbanized, cosmopolitan state that enjoyed the loyalty of its population. In its place rose a set of fragmented, self-sufficient rural domains focused on their own preservation. This shift occurred most dramatically in the Western Empire that formally collapsed in 476 A.D. when the Germanic chief Odoacer was proclaimed Emperor. The Eastern Empire continued for another thousand years as the Byzantine Empire.

The Empire left a powerful legacy to Western civilization. Roman law became the basis of later European legal codes, and the dream of a world state continued to inspire Western rulers. European civilization absorbed the Greek rational tradition largely through Roman cultural achievements. Latin became the common language of learning and the source of several European languages. Finally, Roman universalism shaped the rise of Christianity to its position as the core religion of the West.