Identifying why the western Roman Empire collapsed in 476 AD has been a parlor game for at least two centuries, since Edward Gibbon published his monumental The History of the Decline and Fall of the Roman Empire (Abridged). Gibbon concluded Christianity had a major role in weakening the Empire, a view few today share.
Part of the fun of the parlor game is trying to identify the one thing that pushed it over the cliff: poisoning from lead pipes and wine goblets being a famous example that has been discounted by modern historians.