Evolution has made Homo Sapiens, like other social mammals, a xenophobic creature. Sapiens instinctively divide humanity into two parts, “us” and “them.” “Us” is people like you and me, who share our language, religion, and customs. We are all responsible for each other, but not responsible for “them.” We were always distinct from them, and owe them nothing. We don’t want to see any of them in our territory, and we don’ care an iota what happens in their territory. They are barely even human.
In contrast with this ethnic exclusiveness, imperial ideology from Cyrus onwards has tended to be inclusive and all encompassing.
Empires have played a decisive part in amalgamating many small cultures into fewer big cultures.
Most of today’s cultures are based on imperial legacies.
Since around 200BC, most humans have lived in empires. It seems likely that in the future too most humans will live in one.