Besides conquest by another empire,what other force most often toppled ancient empires?

Question

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Internal conflicts, such as power struggles or rebellions, have been a significant factor in the downfall of many ancient empires. Examples include the fall of the Roman Empire due to political turmoil and the collapse of the Mayan civilization due to internal warfare. Harsh environmental conditions, changes in religion, poor trade relations, and diverse people groups may have played a role as contributing factors, but internal conflicts were often the primary cause of collapse.

Besides Conquest by Another Empire,what Other Force Most Often Toppled