What factors lead to the rise and fall of empires?
Consequently, what causes the downfall of empires?
Often times, the biggest reason for the decline of an empire is that they have become overextended; either by reaching too far, too fast or by being forced, often times through economic need, to keep on capturing land.
Correspondingly, what common factors contributed to the collapse of the Chinese and Roman empires?
While China's increase in population contributed to the collapse of the empire, the decrease of the Roman population contributed to the fall of the Roman empire. Roman's decreased in population was caused by the outbreak of smallpox that was picked up from along the silk road.
The largest contiguous empire in history, the Mongol Empire emerged from the unification of Mongol and Turkish tribes under Genghis Khan. The Mongols achieved advancements in various technologies and ideologies during the empire.