Tarquinius' son was also a terrible leader, as he raped Lucretia, the daughter of a Roman noble, leading to her suicide. In 509 BC, a group of Roman nobles revolted against Tarquinius Superbus, expelling him from power and starting the Roman Republic. This event was significant enough that the word rex (king in Latin) retained a negative connotation throughout the Roman Empire's existence.
The Senate, which had only been advisory during the Roman kingdom, gained more power during the Republic. Several assemblies were created, including the Comitia Centuriata, which elected important officials and decided on matters of war and peace. The Roman Republic's constitution set checks and balances and separated powers.
Around 400 BC, Rome was attacked by the Gauls, who had entered Italy, gone through Etruria, and reached Rome. At the Allia River (10 miles north of Rome), the Gauls defeated the Romans, entered the city, set fire to many buildings (including the royal palace, which housed all the archives that were lost forever), and looted and occupied the city for seven months. The Gaul Invasion of Rome in 390 BC was a significant event that shook the early Roman Republic to its core. According to ancient sources, the Gauls were led by a chieftain named Brennus, who invaded Italy with a massive army. However, the Romans eventually defeated the Gallic army. The invasion had a profound impact on Roman society, and it led to major political and military reforms. The Roman army was reorganized, and several new measures were put in place to strengthen the city's defences. The invasion also reinforced the importance of the Roman Senate, which was given greater powers and responsibilities in the wake of the attack.
After the Gaul Invasion, Rome began a period of rapid expansion, both militarily and territorially. During the period of Roman expansion from 400-200 BC, Rome emerged as a major power in the Mediterranean world. The conquest of Etruria, for example, brought Rome into contact with the powerful city-states of central Italy and helped establish Rome as a regional power. The conquest of Magna Graecia, which included the cities of Tarentum and Sybaris, gave Rome control over important seaports and enabled the city to expand its trade networks.
The expansion of Roman power during this period was fuelled by several factors, including economic incentives, the need for new territory to expand Rome's agricultural production, and a desire for power and prestige among Rome's ruling class. This expansion set the stage for Rome's eventual domination of the Mediterranean world and its emergence as a superpower in the ancient world.