Why were so many tribes able to invade the Western Roman Empire? Full sentences

Invasions by Barbarian tribes

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.

Explanation:

the name of the theory is landmass theory.