What destroyed Western classical civilisation?

When I was taught classical history at school, I was told that the Roman Empire and Western civilisation was destroyed by the barbarian tribes of Germany and central Asia who, during the fourth and fifth centuries swarmed into the Empire and destroyed the political power of the Eternal City of Rome. The migrations of the Goths, Vandals, and Huns were responsible for reducing Europe to an economic and cultural wasteland, and initiating the long period of backwardness we call the 'Dark Ages'.

Dr Bill Warner has a different story to tell. This video is nearly 45 minutes long but do watch it all and wonder why this story is raely, if ever, told.

So why is this sort of exposition of the history of Islamic violence not referred to in our Western education system?