Which was the primary reason that the Western Roman Empire fell?

Losses sustained against outside forces

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. For most of its history, Rome's military was the envy of the ancient world. But during the decline, the makeup of the once mighty legions began to change. Unable to recruit enough soldiers from the Roman citizenry, emperors like Diocletian and Constantine began hiring foreign mercenaries to prop up their armies. The ranks of the legions eventually swelled with Germanic Goths and other barbarians, so much so that Romans began using the Latin word "barbarus" in place of "soldier."