fallen empire

A fallen empire is a nation or civilization that once held great power but has since declined or collapsed.

The phrase evokes lost grandeur—Rome, Persia, the British Empire: We walked through ruins of a fallen empire.

In strategy games, fallen empires are ancient civilizations that have stagnated but retain formidable strength.

The term applies figuratively to any once-dominant entity that has lost its former glory: The company was a fallen empire, a shadow of its 1990s dominance.