Licinius Crassus, Gnaeus Pompeius Magnus, otherwise known as Pompey the Great, and Gaius Julius Caesar, whose name would become synonymous with dictatorial and imperial ambitions and whose assassination by his one-time colleagues became one of the seminal events in human history. These three individuals collectively ruled as the First Triumvirate: in short, a trio of autocratic personalities with little to no regard for the concept of elective government.

While the Roman Senate as an institution had been rendered ineffective and Rome increasingly dominated by Pompey and Caesar, these two military and political leaders turned against each other and became bitter and violent enemies, the former continuing to, at least ostensibly, support the concept of a senate, the latter adamant that he and only he could save Rome. Caesar ultimately prevailed in this conflict, and his accession to the pinnacle of political power in Rome was both absolute and exceedingly temporary, his assassination a sign of the city-state’s continued weariness of one-man rule.

In short, then, the Republic was ruled by a number (originally around 900) of senators whose individualism and conflicting personal and political agendas led to concerns about chaos—concerns that were exploited by Julius Caesar and others whose ascent to power and marked proclivity for military expansionism marked the transition from Republic to Empire. The defining feature of the Republic, however, was the size, composition, and influence of the Senate. As the Senate weakened, the Republic faded, replaced by autocratic rule.

The main difference between the Roman Republic and the Roman Empire concerned the institutions of government. Under the Republic, power was dispersed between the Roman aristocracy and the common people, or plebs. The Senate was the main instrument of power of Roman nobles, whereas the plebs could vote for a tribune to represent their interests. Furthermore, there was no one ruler of Rome; power was shared between two consuls, and both of them were elected to serve one year in office.

In practice, however, the Roman Republic was beset by bitter struggles between powerful aristocrats such as Julius Caesar and Pompey, and it was these struggles that eventually led to the downfall of the Republic and its replacement with the imperial system of government.

Once Octavian—later Augustus—became the first emperor, supreme power was vested in just one man. Octavian was clever in that he didn't openly declare himself as Emperor; the title was conferred upon him by a grateful Senate. He knew what had happened to his uncle, Julius Caesar, when he overreached himself and accrued too much power.

So, at the outset of his lengthy reign, Augustus modestly stated that he would exercise power in accordance with the old traditions. But this was all just for show. In reality, Augustus would come to acquire much more power than Julius Caesar could have dreamed of.

In addition to benefitting from his remarkable ability to hide his true motives, Augustus benefitted from the Roman people's war fatigue. They were fed up of constant strife and wanted peace and stability. And for this, they were prepared to invest Augustus with enormous, almost dictatorial powers.