As the Roman Republic expanded into a world empire, it confronted the limitations of city-state politics. Emerging from a group of farming communities, Rome fell under the influence of the Etruscan city-states of northern Italy and the Greek cities of southern Italy. From the Etruscans the Romans borrowed architectural and engineering techniques, artistic and religious forms, and symbols of authority. Proclaimed in 509 B.C., the oligarchical Republic soon entered the Struggle of the Orders that resulted in greater rights for the plebeians, including the legal code known as the Twelve Tables. In practice, the Republic continued to be dominated by an elite motivated by aristocratic values of virtue, dignity, and fame.

During the Struggle, the Republic began its long period of imperial expansion. By 264 B.C. Rome had fashioned the Italian Confederation, uniting its members by granting them degrees of Roman citizenship and measures of self-government. Now a formidable military power, Rome became involved in Mediterranean politics. Through the three Punic Wars Rome struggled with and ultimately defeated Carthage, thus winning control of the western Mediterranean. Wars with the Hellenistic kingdoms, particularly Macedonia, made Rome master of the eastern Mediterranean. The consequences of this expansion included the development of the jus gentium legal code, the rise of the powerful Equites business class, a taste for luxury among the upper classes, and the importation into Italy of millions of foreign slaves. Conquered peoples often gained no rights of citizenship and suffered under misrule, Roman administration generally improved conditions in the subject lands.

Expansion also brought Rome under the influence of Greek civilization, enabling a flowering of Republican culture. Greek tutors introduced Romans to Hellenic literature, philosophy, science, and medicine. Plautus and Terrence based their popular plays on Menander's New Comedy, and Catullus developed a Latin lyric poetry to articulate his frustrated love for Clodia. Lucretius became the leading Roman Epicurean, advocating withdrawal from public life in response to the civil strife of the late Republic. Cicero, the great orator and statesman, adopted Stoicism, applying its principles to a variety of subjects in brilliant Latin prose.

After the defeat of Carthage and the Hellenistic states, the Republic began its long decline. Without external threats requiring cooperative responses, the remarkable civic spirit of all the classes eroded. In response to an agricultural crisis, the Gracchi led the cause of land reform, angering the Senate and its supporters who killed them. The Republic then degenerated into murderous intrigue and factional strife. The rival generals Marius and Sulla initiated the practice of maintaining private armies with no loyalty to the state. Sulla also restored the old power of the Senate, but it failed to govern effectively. The triumvirate of Caesar, Pompey, and Crassus plotted to seize power. After Crassus' death, the remaining two fell into civil war from which Caesar emerged as dictator. Although Caesar initiated several sound reforms, the ruling elites feared his ambition. After a group of Senators assassinated him, civil war erupted again, leaving Octavian master of Rome and poised to create the Empire.

Unable to move beyond the forms of city-state politics, the Republic proved unable to govern its world empire. The height of Roman rule came later when the Empire achieved effective political and legal universalism.