The passage from monarchy to republic (510-509 BC) was not only a simple institutional change. It also involved a profound juridical and social transformation, as with the emancipation of the plebs who succeeded in gaining access to the highest offices of State, previously a monopoly of the patrician oligarchy. The complex events of the social struggles with the latter class produced the promulgation of written laws for the first time. These Laws of the Twelve Tables, carved in bronze (450 BC), were soon followed by others.
While developing its own institutions and social structure, the Roman State found itself involved in a series of conflicts with the neighbouring populations. Rome so succeeded in strengthening her position that at the end of the 3C BC rivalled the other four great military Mediterranean powers: Carthage, Egypt, Syria and Macedonia.
After having survived the danger of new Gallic invasions, which in 390 BC had crossed the Po Valley and the Apennines to sack Rome itself after having defeated armies first at Chiusi and then on the banks of the Allia (387 BC), Rome completed the conquest of Lazio. It did this by conquering the towns of the Volsci (Anzio) to the south and those of the Etruscans (Tarquinia, Faleri and Caere) to the north of the Tiber; Veio had already been acquired after a ten-year siege at the beginning of the century (396 BC) by Furius Camillus.
In mid-4C BC, following its gradual expansion, Rome necessarily came up against the Samnites who had descended from the heart of the central-southern Apennines towards the fertile lands of Campania, where they rapidly conquered the flourishing towns of Capua (438 BC) and Cuma (421 BC). The rich town of Paestum had already been occupied by the Lucanians. Rome wisely entered an alliance with the Samnites (354 BC) against the pressure of the nearby populations. Conflict with the Samnites for Campanian dominance was however inevitable and lasted for over half a century (343-290 BC). It had three distinct phases with alternating fortunes, such as the crushing Roman defeat at Caudine Forks (321 BC), until Rome won the definitive victory at Sentinum (295 BC) against a coalition that also included Etruscans and Senones, a Gallic tribe.
With her predominance in Central Italy consolidated, Rome prepared to extend it over the rest of the peninsula during a ten-year conflict with Taranto (282-272 BC), who was allied with the king of Epirus, Pyrrhus. While they enjoyed a modest victory at Ausculum (279BC), they were heavily defeated at Beneventum (275 BC). Rome thus achieved total supremacy of the Italian peninsula and set up a complicated system of alliances between the territory of Rome, towns and colonies enjoying full or partial Roman citizenship (`civitates sine suffragio') and the others who, while being independent, recognized Roman sovereignty in the context of a confederation extending over some 130 000 sq km and equipped with well over half a million soldiers Romans and allies).
The economy of the whole Italic federation, whose territory now extended from Tuscany (through Pisa-Pistoia-Fiesole-Rimini, but excluding the upper course of the Arno) to southern-most Calabria, was strengthened by the construction of the first important inland road, the Via Appia, running from Rome to Capua and Benevento (312-268 BC), as well as the development of the fleet and marine transport. At the same time the monetary system was expanded with the minting of bronze (300 BC) and silver (269 BC) coins.