After 500 BC, Rome joined with the Latin cities in defence against incursions by the Sabines. Winning the Battle of Lake Regillus in 493 BC, Rome established again the supremacy over the Latin countries it had lost after the fall of the monarchy. After a lengthy series of struggles, this supremacy became fixed in 393, when the Romans finally subdued the Volsci and Aequi. In 394 BC, they also conquered the menacing Etruscan neighbour of Veii. The Etruscan power was now limited to Etruria itself, and Rome was the dominant city in Latium. In 387 BC, however, Rome was suddenly sacked and burned by invaders coming from Gaul and led by Brennus, who had successfully invaded Etruria. The northern menace was thwarted by consul Furius Camillus, who defeated Brennus at Tusculum soon afterwards.

After that, Rome hastily rebuilt its buildings and went on the offensive, conquering the Etruscans and seizing territory from the Gauls in the north. After 345 BC, Rome pushed south against other Latins. Their main enemy in this quadrant were the fierce Samnites, who heavily defeated the legions in 321. In spite of these and other temporary setbacks, the Romans advanced steadily. By 290 BC, Rome controlled over half of the Italian peninsula. In the 3rd century BC, Rome brought the Greek poleis in the south under its control as well.

According to tradition, Rome became a republic in 509 BC. However, it took a few centuries for Rome to become the great city of popular imagination. By the 3rd century BC, Rome had become the pre-eminent city of the Italian peninsula. During the Punic Wars between Rome and the great Mediterranean empire of Carthage, Rome's stature increased further as it became the capital of an overseas empire for the first time. Beginning in the 2nd century BC, Rome went through a significant population expansion as Italian farmers, driven from their ancestral farmlands by the advent of massive, slave-operated farms called latifundia, flocked to the city in great numbers. In 146 BC, the Romans razed the cities of Carthage and Corinth, adding North Africa and Greece to its empire and making Rome the most important city in the western world. From this point until the end of the Republic, individual citizens would compete to enhance their personal prestige by erecting monuments and great structures for public use around the city. Most notable was the Theatre of Pompey, erected by the great general Gnaeus Pompeius Magnus, which was the first permanent theatre built in the city. After Caesar emerged victorious from his Gallic conquests and subsequent civil war with Pompey, he embarked on a building program unprecedented in Roman history. He was assassinated in 44 BC, however, with most of his projects, such as the Basilica Iulia and a new Senate house (Curia), still under construction.