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According to several thinkers (Turchin, Travin) the success of empires is driven by their ability to create a multi-ethnic system that agrees on a single set of ideas. Examples: Romans, Mongols, Russian Empire, USSR.
And vice versa, empires fall when the consensus on these common (but not uniform!) values fails, e.g. by nationalism of one group. Example: Polish-Lithuanian Commonwealth.
Or when not everyone benefits equally any more, e.g. USSR.