In the place of the King, two chief executives were chosen annually by the whole body of citizens. These were known as praetors or leaders, but later received the title of consuls. The participation of a colleague in the exercise of supreme power and limitation of the tenure to one year prevented the chief magistrate from becoming autocratic. The character of the Senate was altered by the enrollment of the plebeian members, known as conscripti, and hence the official designation of the senators thereafter was patres conscripti (conscript fathers) . As yet, only patricians were eligible for the magistracies, and the discontent of the plebs led to a violent struggle between the two orders and the gradual removal of the social and political disabilities under which the plebs had labored.

In 494 B. C. a secession of plebeian soldiers led to the institution of the tribune plebes, who were elected annually as protectors of the plebs; they had the power to veto the acts of patrician magistrates, and thus served as the leaders of the plebs in the struggles with the patricians. The appointment of the decemvirate, a commission of ten men, in 451 B. C. resulted in the drawing up of a famous code of laws. In 445 B. C. , under the Canuleian law, marriages between patricians and members of the plebs were declared legally valid. By the Licinuian- Sextian laws, passed in 367 B. C. , it was provided that one of the two consuls should thenceforth be plebeian. The other magistracies were gradually opened to the plebs: in 365 B. C. the dictatorship, an extraordinary magistracy, the incumbent of which was appointed in times of great danger; in 350 B. C. , the censorship; in 337 B. C. , the praetorship; and in 300 B. C. , the pontifical and augural colleges.

These political changes gave rise to a new aristocracy, composed of patrician and wealthy plebeian families, and admission to the Senate became almost the hereditary privilege of these families. The Senate, which had originally possessed little administrative power, became a powerful governing body, dealing with matters of war and peace, foreign alliances, the founding of colonies, and the handling of the state finances. The rise of this new nobilitas brought to an end the struggles between the two orders, but the position of the poorer plebeian families was not improved, and the marked contrast between the conditions of the rich and the poor led to struggles in the later Republic between the aristocratic party and the popular Party.

The external history of Rome during this period was chiefly military. Rome had acquired the leadership of Latium before the close of the regal period. Assisted by their allies, the Romans fought wars against the Etruscans, the Volscians, and the Aequians. The military policy of Rome became more aggressive in the 60 years between 449 and 390 B. C. The defeat of the Romans at Allia and the capture and burning of Rome by the Gauls under the leadership of the chieftain Brennus in 390 B. C. were great disasters, but their effect was temporary. The capture of the Etruscan city of Veii in 396 B. C. by the solider and statesman Marcus Furius Camillus spelled the beginning of the end of Etruscan independence. Other Etruscan cities hastened to make peace, and by the middle of the 4th B. C. all southern Etruria was kept in the check by Roman garrisons and the denationalized by an influx of Roman colonists. Vistories over the Volscians, the Latins, and the Hernicans gave Romans control of central Italy and brought them into conflict with the Samnites of southern Italy, who were defeated in a series of three wars, extending from 343 to 290 B. C. A powerful coalition was at this time formed against Rome, consisting of Etruscans, Umbrians, and Gauls in the north, and of Lucanians, Bruttians, and Samnites in the south; this coalition endangered the power of Rome, but the northern confederacy was defeated in 283 B. C. and the southern states soon after.