There is something peculiar to the civilizations. There are usually two peoples which make up a civilization. A junior and a senior nation. Usually it is the junior partner of an earlier civilization which becomes the senior partner of the newer civilization, combined with the newly arrived peoples in a Völkerwanderung.
Prior to the establishment of a universal state you get a battle between two countries- one based on culture and trade and the other on military, only to be defeated by the fake unificator - a semi barbarian which wished to unify the civilization but didn't do so in the long term.
Ultimately, the unification is brought upon by the junior partner.
See the Sumerian civilization being composed of the senior Sumerians and the elder Akkadian. The Aegean /Minoan with a senior Minoans and junior Myceneans. The Syriac with a senior Semites and junior Iranians and the Hellenistic with senior Greeks and junior Latins.
The Western has a Latin senior and a Germanic junior partner. The Byzantine has a senior Greek and junior Slavic partner. Similarly the Persian civilization has a senior Iranian and a junior Turkic partner.