Answer:
In the political differences between a republic and an empire.
Explanation:
The Roman Republic, that lasted from 509 BC to 27 BC, was one of the most succesful expressions of Democracy in ancient times. Every class (except of course slaves) felt represented. Through a combination of military power, economic expansion and political flexibility, Rome became the most powerful state in the world. However, in the end the Republic was severely destabilized by civil wars. Julius Ceasar was, de facto, a dictator and one of the last to rule the Roman Republic.
The Roman empire, therefore, established itself when the Roman Senate granted Octavian, Ceasar´s adopted son, overarching power and the new title Augustus, effectivily making him the first Roman Emperor. From then on the Pax Romana stabilized the Empire during two centuries.
It was exactly during this period of stability and prosperity that the Roman Empire reached it´s greatest extent, in 117 AD.
