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2) Decadence -- gradual loss of the infrastructure or know-how that allowed the empire to expand (historically, often a communication and transportation network that enabled supervisor military command & control)
3) Internal Strife -- for example the empires of Alexander and Genghis Khan quickly disintegrating into competing factions during succession wars
4) The Hammer of Fate -- pandemics, earthquakes, fires, volcanic eruptions... In particular, plagues that wipe out 20-50% of the population seem to be a common fixture of the fall of great empires, acting as a trigger or a compounding factor
3) is more "peaceful and formalized transition of power".
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