Why would multiple groups attacking the Western Roman Empire at the same time be more challenging to defend against?

"Fall of Rome" redirects here. For other uses, see Fall of Rome (disambiguation).

"The Fall of the Roman Empire" redirects here. For the film, see The Fall of the Roman Empire (film).

Explanation:

the steam engine.

the locomotive.

the telephone.

the telegraph.

the internal combustion engine.

the rifle.

ironclad ships.

electricity/light bulb.