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Are there any similarities in what happens when Empires collapse?

The question I have is 2 fold what causes most Empires or Nations to fall other than over powered by a superior foe? Is there anyway to prevent an empire or nation from falling once events are set in place?

Pretty much the main reasons for a nation or empire falling is civil strife, economic collapse, population collapse or cultural shock (and any others). I want to know what seems to be the most common.

The second part is more once a nation/empire begins to fall is there anything to stop it or is it human nature to accept the destruction as a unchangeable event.