The Roman state from the overthrow of the monarchy (traditionally 509 BCE) to the principate of Augustus (27 BCE), governed by annual magistrates, a Senate and popular assemblies, and analysed by Polybius as a mixed constitution.
By tradition the Romans expelled their last king in 509 BCE and chose two annually elected consuls instead; the early record is poor and later embellished. Magistrates led armies and administered the state, the Senate advised them and in practice held great power, and citizens voted in assemblies organised by military class (centuriate) or by tribe, which elected magistrates, passed laws and tried serious offences. Tribunes of the plebs could protect citizens and veto measures, and a dictator could be named in emergencies for at most six months. Only male citizens voted, and slavery was central to the economy. Civil wars in the late republic ended with Octavian's settlement of 27 BCE.
Produced durable institutions of annual office, collegiality and advisory senatorial government, and a first written code (the Twelve Tables, 451 BCE). Polybius' account of its mixed constitution became a standard reference in later political thought.
Imperial expansion and civil wars brought powerful military commanders. After Caesar's dictatorship and assassination (44 BCE) and Octavian's victory at Actium (31 BCE), Octavian received the name Augustus in 27 BCE and the principate replaced the republic. Causes are debated and multiple.
The republic's core institutions were two annually elected consuls, a Senate that advised magistrates and in practice held great power, and popular assemblies (centuriate and tribal) that elected magistrates and voted on laws; tribunes of the plebs could veto.[1][2][3]
Polybius described Rome's constitution as combining monarchical (consuls), aristocratic (Senate) and democratic (people) elements that could check or support one another, and argued that such a combination is the best form of constitution.[1]
The popular vote was central to Roman republican politics, making the republic meaningfully democratic.[1][2]
The republic ended after civil wars: Caesar took power as dictator and was assassinated in 44 BCE, Octavian won at Actium in 31 BCE, and in 27 BCE Octavian received the name Augustus and the title princeps.[1]