Bands -Tribes - Nations - Empires
That's how humans organize themselves
Bands are small communities, typically no more than a couple of hundred people, almost always related by blood and where everybody personally knows everybody else. Leadership may be by consensus or by unofficially acknowledging someone in the group as the leader.
Tribes are related mostly along lines of kinship, and while larger than bands by an order of magnitude, people in a tribe know of everybody else in it, and relationships and reputations can be swiftly determined. Leadership may be loosely traditional or formally codified, but generally is older, experienced members of the tribe.
Nations are comprised of many different tribes, but almost always linked by culture / language and usually by geography (even nomadic nations are known by the territory they roam). Members of a nation know about others in the nation, but not necessarily direct first hand knowledge. Leadership is typically formally codified.
Empires are an amalgam of many different nations / cultures / languages. Typically formed by conquest and colonization, if run well an empire justifies its existence through mutual support / defense / benefit. Members of an empire do not know all the different tribes / nations / cultures that make it, and indeed empires can often contain groups strongly antithetical to one another; the great virtue of an empire is that it can tamp down these antagonisms through objective codified justice and leadership through which all groups feel they are treated fairly.
(The above is derived from Jared Diamond's Guns, Germs, And Steel: Highly recommended.)
The United States stopped being a nation and became an empire in the wake of the Mexican War, our first conquest of another group's territory that absorbed the members of that group instead of annihilating them or driving them away. It was cemented as an empire in the wake of massive non-Anglo immigration and the emancipation of African-American slaves (slaves in general not being considered citizens but property undeserving of rights or respect).
This country was built on the theft of land and labor by whites to the exclusion of others. Out of necessity America was forced to include non-whites, but it never fully came to terms with them the way Rome came to terms with non-Italian peninsula citizens or Byzantium with its plethora of cultures or even the Islamic world at the height of the caliphate.
We are paying for that refusal to come to terms.