Kull AI · with Library sources

There's no single cause — historians point to a compounding set. The short version:

Overextension & cost. The empire grew faster than it could defend, and the army became ruinously expensive along thousands of miles of frontier.

Political instability. Rapid turnover of emperors, civil wars, and weakening central authority eroded continuity.

Economic strain. Currency debasement, heavy taxation, and disrupted trade hollowed out the tax base.

External pressure. Sustained migration and invasion finally broke the western half in 476 AD.