What was the Crippling Blow that destroyed the Western Roman Empire?

German emperor attacked Italy, and made himself king.

Explanation:

The attack itself is not the main reason for Roman's downfall. Western Roman empire gradually became weaker after its territory was continually divided during the power struggles between the King's children.

If the attack came to Rome during the height of its  power. The German's would be easily defeated.

Odoacer , One of The German emperors at that time, attacked the Italian  territory while they're at the weakest point. He declared himself as King Italy right after and marked the downfall of the western Roman empire.

answer; a splinter party;