Why did the Roman Empire decline and then fall?

Invasions by Barbarian tribes

Explanation: The most straightforward theory for Western Rome's collapse pins the fall on a string of military losses sustained against outside forces. Rome had tangled with Germanic tribes for centuries, but by the 300s “barbarian” groups like the Goths had encroached beyond the Empire's borders.

its repetition emphasizing that the chain is his own. sorry if i am wrong.

yes, they are. i think.