1. The decline and fall of the Roman empire.
Many causes have been advocated. I would add one (I'm not sure it is original): Due to Christianity capable generals in the 5th century like Aetius, Stilicho and Belisarius did not revolt as easily as the ones from the 3rd century. Therefore incapable emperors like Honorius stayed in power and sometimes murdered their generals. This weakened the empire.

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