Answer: Invasions by Barbarian tribes. Economic troubles and overreliance on slave labor. Overexpansion and military overspending.
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. Rome fell through a gradual process because poor economic policies led to a weakened military which allowed the barbarians easy access to the empire. In the third century, Rome's emperors embraced harmful economic policies which led to Rome's decline.