The Roman Empire was a nation state on Earth, ruled by an emperor, early in its history. It was copied in its development on the planet 892-IV (known as Magna Roma) except that this planet went on to develop like the Earth's 20th century while the Empire had fallen in the 400s AD.
The Roman Empire also inspired Isaac Asimov to write his Foundation series. There, we find the Galactic Empire which is modeled like the original Roman Empire. The capital is Trantor, and there is a saying that all paths lead to it, like the ancient saying about Rome.
For historical information go to Roman Empire at Wikipedia