The Roman Republic was governed by a complex constitution, which centered on the principles of a separation of powers and checks and balances. The evolution of the constitution was heavily influenced by the struggle between the aristocracy, or the patricians, and other talented Romans who were not from famous families, the plebians. Early in its history, the republic was controlled by an aristocracy of individuals who could trace their ancestry back to the early history of the kingdom. Over time, the laws that allowed these individuals to dominate the government were repealed, and the result was the emergence of a new aristocracy which depended on the structure of society, rather than the law, to maintain its dominance. Thus, only a revolution could overthrow this new aristocracy.
The precise event which signaled the end of the Roman Republic and the transition into the Roman Empire is a matter of interpretation. Towards the end of the period a selection of Roman leaders came to so dominate the political arena that they exceeded the limitations of the Republic as a matter of course. Historians have variously proposed the appointment of Julius Caesar as perpetual dictator in 44 BC, the defeat of Mark Antony at the Battle of Actium in 31 BC, and the Roman Senate's grant of extraordinary powers to Octavian under the first settlement in 27 BC, as candidates for the defining pivotal event ending the Republic.
Can be found at:
http://en.wikipedia.org/wiki/Roman_Republic
