Nation

On a casual look, the two empires are rather similar, although they are entirely different in reality.

(The only similarity is both of them adopted remarkably modern political systems. That’s another story.)

You see? The Han Empire represented the idea of nationalism. People of the same race, Han (漢族) in this case, live together and form a country. Citizens are bound together because they share the same genes as well as culture and tradition. Settlers at the border are barbarians and should be conquered – they could, and would, never be incorporated into the country and become “one of us”. On the other hand, if the people is living under the ruling of a different race, they would fight for independence.

In contrast, the Roman Empire represented a country of idea. The territory was huge, and there were people from all races. Nonetheless, they are unified by having the same thinking – value, idea, dream, motto, religion, whatever you call it. As long as one accepts the core value or thinking, they could become a citizen of the Roman Empire, irrespective to their skin colour and genetic background.