What event or development caused a final decline of Roman authority in the Western empire, starting about the year 400?

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A new wave of barbarian invasions caused a final decline of Roman authority in the Western empire starting around 400 AD. The barbarian tribes such as the Visigoths, Vandals, and Huns began to invade and conquer Roman territory, weakening the empire's military and political power. Eventually, these tribes were able to sack Rome itself and effectively end the Western Roman Empire.

What Event or Development Caused a Final Decline of Roman