The Roman Republic, formed from the old Roman Kingdom in 509 BC, was a nation and then empire that eventually expanded over the entire Mediterranean by conquering less advanced peoples. Rome was the premier economic and military power in all of Europe and Africa by 148 BC, following the defeat of Carthage and the Greek Kingdoms. The Republic and its successor state, the Roman Empire, would maintain this position of local superpower until the collapse of the empire in 317 AD.
The Roman Republic eventually declined due to a lengthy process which annihilated its political system that began with Tiberius Gracchus. Gracchus and his brother, Gaius Gracchus, would significantly weaken the senate and increase Plebian power. This weakness in the senate was capitalized upon by the legate Gaius Marius. After his defeat of the Numidians, which showed Marius's strength and showed senate incompetence, Marius was elected consul, supported by the Populares. When his main political opponent and extreme conservative Lucius Sulla was dispatched to Pontus to defeat the uprising of King Mithridites, Marius and his supporters took control of the city of Rome. Sulla, upon hearing this, made peace with MIthridites and invaded his own city, killing Marius and establishing himself as dictator. Following Sulla's resignation, two of his lieutenants, Gnaeus Pompeius Magnus and Marcus Licinius Crassus joined the Populares party and once again stole the reigns of power away from the rest of the Senate, although this time solely for their own ambition. Following the death of Crassus in Parthia and the defeat of the other main political force in Rome, Gaius Julius Caesar, in his Civil War, Pompey became the dictator of Rome. Following his demise, Rome effectively became a dictatorship, as neither the senate nor the Plebians were able to retain power over the army. Eventually, in the year 21 BC, Lucius Julius Brutus, upon his conquering of Assyria, was declared Dictator for Life and Imperator, ending the Roman Republic and beginning the Roman Empire.
List of Dictators of Rome (From Caesar's Civil War onward)