The Romans established a system of annually elected magistrates and various representative assemblies.  Leaders provided a constitution of checks and balances and a separation of powers.  Most important, there were two consuls who together exercised executive authority in the form of imperium, or military command.
A number of magistracies (offices) were established and filled by patricians (elite).  Later, these offices were opened to the common people, or plebeians.  Later this classification would become the source of severe, political  confrontations.
By now, Rome was an important force on the Italian peninsula.  A treaty with Carthage, an Empire on the sourthern shore of the Mediterranean Ocean confirmed this standing.  Tribes near Rome because of apprehension of its developing power formed with the Etruscans the Latin League as a counterbalance to this threat.  The League shared rights of commerce, inter-marriage and settlement with its citizens.  These crucial rights and other rights would constitute key elements in future Roman politics, negotiations and treaties among city-states, provinces, provinces and tribes for centuries to come. 
When Etruscan rule of Rome was broken, The League vied with other regional tribes for dominance.  The balance of power often shifted between Rome and other influential cities like Alba Longa and Lavinium.  By 496 BC, these power grabs turned to war.  Some tribes moved against Rome and resulted in a stalemate showing that Rome could stand against their combined power.  The war drew to a close in 493 BC, but the continuing external pressures on these tribes by the Umbrians (Volsci and Aegui) and Sabines forced Rome’s immediate neighbors into closer ties and alliances with Rome.
The Latin towns of Tibur and Praeneste in the east were eliminated by the Aequi.  A Roman army was destroyed at the Agidus Pass.  The Volsci were in control of south Latium (Cora, Velitrae, Satricum), and pressured the Latium from there.  The Romans under General Tuibertus again met the Aegui at Agidus Pass and defeated them.  By 390 BC, the Romans Legions had driven the Aegui and the Volsci (the Umbrians) into the highlands.  They were finally defeated at Antium 377 BC.
Northeast of Rome (the territory between the Tiber and Anio Rivers) was being constantly pressured by the attacks of the Sabines.  A major Roman victory in 449 BC resulted in the absorption of the Sabines into the Roman Republic.   Offering some rights for the conquered but no rights to vote were examples of the Roman strategy for “Romanizing” the conquered states into and within the Republic.  With victories over the Umbrians and the Sabines, the military policy of Rome became much more aggressive in the 60 years 449 – 390 BC.  The Etruscans still remained a constant threat.  By 396 BC, in large part due to the savagery of the Gothic invasions, the Etruscans were vulnerable to conquest.  By 273 BC, Eturia and the Etruscans were completely absorbed into the Roman Republic.
Roman expansion to the south was to be met by the Samnites (Greek) in a series of several wars.  In the 300 years since its founding, Rome had yet to secure its own region of Latium and still found itself facing foreboding challenges.