Why do you think the two
empires did not last very long?

1. 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.

2. One of the biggest reasons for the decline of the empires is that they have become overextended; either by reaching too far, too fast or by being forced, often times through economic need, to keep on capturing land.

Explanation: