Why have historians called the Assyrian Empire of the first millennium B.C.E. the first true empire?

Question

Why have historians called the Assyrian Empire of the first millennium B.C.E. the first true empire?

Answer 1

I'm not sure, but I'm here to tell you Assyrians still exist!!

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Answer 2

Because of the masses of land, the several countries, that was conquered and turned into a multi ethnic and centrally ruled empire.

An empire is a state that extends dominion over populations distinct culturally and ethnically from the culture/ethnicity at the center of power.