Final answer:
Barbarian tribes weakened the Roman Empire's fragile state.
Step-by-step explanation:
The external force that weakened the Roman Empire's fragile state was Barbarian tribes. These tribes, such as the Visigoths, Ostrogoths, and Vandals, launched invasions and attacks on the empire, gradually eroding its power and control over its territories.
One significant event that marks the weakening of the Roman Empire is the sack of Rome in 410 CE by the Visigoths under the leadership of Alaric. This attack demonstrated the vulnerability of Rome and the declining ability of the empire to defend itself against external threats.
Another notable event is the fall of the Western Roman Empire in 476 CE. This event was triggered by the invasion of Germanic chieftain Odoacer, leading his army of Heruli, Rugians, and Scirians who deposed the last Western Roman Emperor, Romulus Augustulus.
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